DESCRIPTION
This page describes the syntax of regular expressions in Perl.
If you haven't used regular expressions before, a quick-start
introduction is available in perlrequick, and a longer tutorial
introduction is available in perlretut.
For reference on how regular expressions are used in matching
operations, plus various examples of the same, see discussions of
"m//", "s///", "qr//" and "??" in "Regexp Quote-Like Operators" in
perlop.
Modifiers
Matching operations can have various modifiers. Modifiers that relate
to the interpretation of the regular expression inside are listed
below. Modifiers that alter the way a regular expression is used by
Perl are detailed in "Regexp Quote-Like Operators" in perlop and "Gory
details of parsing quoted constructs" in perlop.
m Treat string as multiple lines. That is, change "^" and "$" from
matching the start or end of the string to matching the start or
end of any line anywhere within the string.
s Treat string as single line. That is, change "." to match any
character whatsoever, even a newline, which normally it would not
match.
Used together, as /ms, they let the "." match any character
whatsoever, while still allowing "^" and "$" to match,
respectively, just after and just before newlines within the
string.
i Do case-insensitive pattern matching.
If "use locale" is in effect, the case map is taken from the
current locale. See perllocale.
x Extend your pattern's legibility by permitting whitespace and
comments.
p Preserve the string matched such that ${^PREMATCH}, {$^MATCH}, and
${^POSTMATCH} are available for use after matching.
g and c
Global matching, and keep the Current position after failed
matching. Unlike i, m, s and x, these two flags affect the way the
regex is used rather than the regex itself. See "Using regular
expressions in Perl" in perlretut for further explanation of the g
and c modifiers.
These are usually written as "the "/x" modifier", even though the
delimiter in question might not really be a slash. Any of these
modifiers may also be embedded within the regular expression itself
Perl's regular expressions more readable. Note that you have to be
careful not to include the pattern delimiter in the comment--perl has
no way of knowing you did not intend to close the pattern early. See
the C-comment deletion code in perlop. Also note that anything inside
a "\Q...\E" stays unaffected by "/x".
Regular Expressions
Metacharacters
The patterns used in Perl pattern matching evolved from those supplied
in the Version 8 regex routines. (The routines are derived (distantly)
from Henry Spencer's freely redistributable reimplementation of the V8
routines.) See "Version 8 Regular Expressions" for details.
In particular the following metacharacters have their standard
egrep-ish meanings:
\ Quote the next metacharacter
^ Match the beginning of the line
. Match any character (except newline)
$ Match the end of the line (or before newline at the end)
| Alternation
() Grouping
[] Character class
By default, the "^" character is guaranteed to match only the beginning
of the string, the "$" character only the end (or before the newline at
the end), and Perl does certain optimizations with the assumption that
the string contains only one line. Embedded newlines will not be
matched by "^" or "$". You may, however, wish to treat a string as a
multi-line buffer, such that the "^" will match after any newline
within the string (except if the newline is the last character in the
string), and "$" will match before any newline. At the cost of a
little more overhead, you can do this by using the /m modifier on the
pattern match operator. (Older programs did this by setting $*, but
this practice has been removed in perl 5.9.)
To simplify multi-line substitutions, the "." character never matches a
newline unless you use the "/s" modifier, which in effect tells Perl to
pretend the string is a single line--even if it isn't.
Quantifiers
The following standard quantifiers are recognized:
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
(If a curly bracket occurs in any other context, it is treated as a
regular character. In particular, the lower bound is not optional.)
a "?". Note that the meanings don't change, just the "greediness":
*? Match 0 or more times, not greedily
+? Match 1 or more times, not greedily
?? Match 0 or 1 time, not greedily
{n}? Match exactly n times, not greedily
{n,}? Match at least n times, not greedily
{n,m}? Match at least n but not more than m times, not greedily
By default, when a quantified subpattern does not allow the rest of the
overall pattern to match, Perl will backtrack. However, this behaviour
is sometimes undesirable. Thus Perl provides the "possessive"
quantifier form as well.
*+ Match 0 or more times and give nothing back
++ Match 1 or more times and give nothing back
?+ Match 0 or 1 time and give nothing back
{n}+ Match exactly n times and give nothing back (redundant)
{n,}+ Match at least n times and give nothing back
{n,m}+ Match at least n but not more than m times and give nothing back
For instance,
'aaaa' =~ /a++a/
will never match, as the "a++" will gobble up all the "a"'s in the
string and won't leave any for the remaining part of the pattern. This
feature can be extremely useful to give perl hints about where it
shouldn't backtrack. For instance, the typical "match a double-quoted
string" problem can be most efficiently performed when written as:
/"(?:[^"\\]++|\\.)*+"/
as we know that if the final quote does not match, backtracking will
not help. See the independent subexpression "(?>...)" for more details;
possessive quantifiers are just syntactic sugar for that construct. For
instance the above example could also be written as follows:
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
Escape sequences
Because patterns are processed as double quoted strings, the following
also work:
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\a alarm (bell) (BEL)
\e escape (think troff) (ESC)
\033 octal char (example: ESC)
\x1B hex char (example: ESC)
\x{263a} long hex char (example: Unicode SMILEY)
You cannot include a literal "$" or "@" within a "\Q" sequence. An
unescaped "$" or "@" interpolates the corresponding variable, while
escaping will cause the literal string "\$" to be matched. You'll need
to write something like "m/\Quser\E\@\Qhost/".
Character Classes and other Special Escapes
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
\W Match a non-"word" character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a digit character
\D Match a non-digit character
\pP Match P, named property. Use \p{Prop} for longer names.
\PP Match non-P
\X Match eXtended Unicode "combining character sequence",
equivalent to (?>\PM\pM*)
\C Match a single C char (octet) even under Unicode.
NOTE: breaks up characters into their UTF-8 bytes,
so you may end up with malformed pieces of UTF-8.
Unsupported in lookbehind.
\1 Backreference to a specific group.
'1' may actually be any positive integer.
\g1 Backreference to a specific or previous group,
\g{-1} number may be negative indicating a previous buffer and may
optionally be wrapped in curly brackets for safer parsing.
\g{name} Named backreference
\k<name> Named backreference
\K Keep the stuff left of the \K, don't include it in $&
\v Vertical whitespace
\V Not vertical whitespace
\h Horizontal whitespace
\H Not horizontal whitespace
\R Linebreak
A "\w" matches a single alphanumeric character (an alphabetic
character, or a decimal digit) or "_", not a whole word. Use "\w+" to
match a string of Perl-identifier characters (which isn't the same as
matching an English word). If "use locale" is in effect, the list of
alphabetic characters generated by "\w" is taken from the current
locale. See perllocale. You may use "\w", "\W", "\s", "\S", "\d", and
"\D" within character classes, but they aren't usable as either end of
a range. If any of them precedes or follows a "-", the "-" is
understood literally. If Unicode is in effect, "\s" matches also
"\x{85}", "\x{2028}", and "\x{2029}". See perlunicode for more details
about "\pP", "\PP", "\X" and the possibility of defining your own "\p"
and "\P" properties, and perluniintro about Unicode in general.
"\R" will atomically match a linebreak, including the network line-
ending "\x0D\x0A". Specifically, is exactly equivalent to
# this is correct:
$string =~ /[[:alpha:]]/;
# this is not, and will generate a warning:
$string =~ /[:alpha:]/;
The available classes and their backslash equivalents (if available)
are as follows:
alpha
alnum
ascii
blank [1]
cntrl
digit \d
graph
lower
print
punct
space \s [2]
upper
word \w [3]
xdigit
[1] A GNU extension equivalent to "[ \t]", "all horizontal whitespace".
[2] Not exactly equivalent to "\s" since the "[[:space:]]" includes
also the (very rare) "vertical tabulator", "\ck" or chr(11) in
ASCII.
[3] A Perl extension, see above.
For example use "[:upper:]" to match all the uppercase characters.
Note that the "[]" are part of the "[::]" construct, not part of the
whole character class. For example:
[01[:alpha:]%]
matches zero, one, any alphabetic character, and the percent sign.
The following equivalences to Unicode \p{} constructs and equivalent
backslash character classes (if available), will hold:
[[:...:]] \p{...} backslash
alpha IsAlpha
alnum IsAlnum
ascii IsASCII
blank
cntrl IsCntrl
digit IsDigit \d
graph IsGraph
lower IsLower
print IsPrint (but see [2] below)
[1] If the "utf8" pragma is not used but the "locale" pragma is, the
classes correlate with the usual isalpha(3) interface (except for
"word" and "blank").
But if the "locale" or "encoding" pragmas are not used and the
string is not "utf8", then "[[:xxxxx:]]" (and "\w", etc.) will not
match characters 0x80-0xff; whereas "\p{IsXxxxx}" will force the
string to "utf8" and can match these characters (as Unicode).
[2] "\p{IsPrint}" matches characters 0x09-0x0d but "[[:print:]]" does
not.
[3] "[[:punct::]]" matches the following but "\p{IsPunct}" does not,
because they are classed as symbols (not punctuation) in Unicode.
"$" Currency symbol
"+" "<" "=" ">" "|" "~"
Mathematical symbols
"^" "`"
Modifier symbols (accents)
The other named classes are:
cntrl
Any control character. Usually characters that don't produce
output as such but instead control the terminal somehow: for
example newline and backspace are control characters. All
characters with ord() less than 32 are usually classified as
control characters (assuming ASCII, the ISO Latin character sets,
and Unicode), as is the character with the ord() value of 127
("DEL").
graph
Any alphanumeric or punctuation (special) character.
print
Any alphanumeric or punctuation (special) character or the space
character.
punct
Any punctuation (special) character.
xdigit
Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f]
would work just fine) it is included for completeness.
You can negate the [::] character classes by prefixing the class name
with a '^'. This is a Perl extension. For example:
POSIX traditional Unicode
[[:^digit:]] \D \P{IsDigit}
\b Match a word boundary
\B Match except at a word boundary
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
\G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
A word boundary ("\b") is a spot between two characters that has a "\w"
on one side of it and a "\W" on the other side of it (in either order),
counting the imaginary characters off the beginning and end of the
string as matching a "\W". (Within character classes "\b" represents
backspace rather than a word boundary, just as it normally does in any
double-quoted string.) The "\A" and "\Z" are just like "^" and "$",
except that they won't match multiple times when the "/m" modifier is
used, while "^" and "$" will match at every internal line boundary. To
match the actual end of the string and not ignore an optional trailing
newline, use "\z".
The "\G" assertion can be used to chain global matches (using "m//g"),
as described in "Regexp Quote-Like Operators" in perlop. It is also
useful when writing "lex"-like scanners, when you have several patterns
that you want to match against consequent substrings of your string,
see the previous reference. The actual location where "\G" will match
can also be influenced by using "pos()" as an lvalue: see "pos" in
perlfunc. Note that the rule for zero-length matches is modified
somewhat, in that contents to the left of "\G" is not counted when
determining the length of the match. Thus the following will not match
forever:
$str = 'ABC';
pos($str) = 1;
while (/.\G/g) {
print $&;
}
It will print 'A' and then terminate, as it considers the match to be
zero-width, and thus will not match at the same position twice in a
row.
It is worth noting that "\G" improperly used can result in an infinite
loop. Take care when using patterns that include "\G" in an
alternation.
Capture buffers
The bracketing construct "( ... )" creates capture buffers. To refer to
the current contents of a buffer later on, within the same pattern, use
\1 for the first, \2 for the second, and so on. Outside the match use
"$" instead of "\". (The \<digit> notation works in certain
circumstances outside the match. See the warning below about \1 vs $1
for details.) Referring back to another part of the match is called a
backreference.
5.10.0). The curly brackets are optional, however omitting them is less
safe as the meaning of the pattern can be changed by text (such as
digits) following it. When N is a positive integer the "\g{N}" notation
is exactly equivalent to using normal backreferences. When N is a
negative integer then it is a relative backreference referring to the
previous N'th capturing group. When the bracket form is used and N is
not an integer, it is treated as a reference to a named buffer.
Thus "\g{-1}" refers to the last buffer, "\g{-2}" refers to the buffer
before that. For example:
/
(Y) # buffer 1
( # buffer 2
(X) # buffer 3
\g{-1} # backref to buffer 3
\g{-3} # backref to buffer 1
)
/x
and would match the same as "/(Y) ( (X) \3 \1 )/x".
Additionally, as of Perl 5.10.0 you may use named capture buffers and
named backreferences. The notation is "(?<name>...)" to declare and
"\k<name>" to reference. You may also use apostrophes instead of angle
brackets to delimit the name; and you may use the bracketed "\g{name}"
backreference syntax. It's possible to refer to a named capture buffer
by absolute and relative number as well. Outside the pattern, a named
capture buffer is available via the "%+" hash. When different buffers
within the same pattern have the same name, $+{name} and "\k<name>"
refer to the leftmost defined group. (Thus it's possible to do things
with named capture buffers that would otherwise require "(??{})" code
to accomplish.)
Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
/(.)\1/ # find first doubled char
and print "'$1' is the first doubled character\n";
/(?<char>.)\k<char>/ # ... a different way
and print "'$+{char}' is the first doubled character\n";
/(?'char'.)\1/ # ... mix and match
and print "'$1' is the first doubled character\n";
if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
Several special variables also refer back to portions of the previous
perlsyn.)
NOTE: Failed matches in Perl do not reset the match variables, which
makes it easier to write code that tests for a series of more specific
cases and remembers the best match.
WARNING: Once Perl sees that you need one of $&, "$`", or "$'" anywhere
in the program, it has to provide them for every pattern match. This
may substantially slow your program. Perl uses the same mechanism to
produce $1, $2, etc, so you also pay a price for each pattern that
contains capturing parentheses. (To avoid this cost while retaining
the grouping behaviour, use the extended regular expression "(?: ... )"
instead.) But if you never use $&, "$`" or "$'", then patterns without
capturing parentheses will not be penalized. So avoid $&, "$'", and
"$`" if you can, but if you can't (and some algorithms really
appreciate them), once you've used them once, use them at will, because
you've already paid the price. As of 5.005, $& is not so costly as the
other two.
As a workaround for this problem, Perl 5.10.0 introduces
"${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", which are equivalent
to "$`", $& and "$'", except that they are only guaranteed to be
defined after a successful match that was executed with the "/p"
(preserve) modifier. The use of these variables incurs no global
performance penalty, unlike their punctuation char equivalents, however
at the trade-off that you have to tell perl when you want to use them.
Backslashed metacharacters in Perl are alphanumeric, such as "\b",
"\w", "\n". Unlike some other regular expression languages, there are
no backslashed symbols that aren't alphanumeric. So anything that
looks like \\, \(, \), \<, \>, \{, or \} is always interpreted as a
literal character, not a metacharacter. This was once used in a common
idiom to disable or quote the special meanings of regular expression
metacharacters in a string that you want to use for a pattern. Simply
quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
(If "use locale" is set, then this depends on the current locale.)
Today it is more common to use the quotemeta() function or the "\Q"
metaquoting escape sequence to disable all metacharacters' special
meanings like this:
/$unquoted\Q$quoted\E$unquoted/
Beware that if you put literal backslashes (those not inside
interpolated variables) between "\Q" and "\E", double-quotish backslash
interpolation may lead to confusing results. If you need to use
literal backslashes within "\Q...\E", consult "Gory details of parsing
quoted constructs" in perlop.
Extended Patterns
Perl also defines a consistent extension syntax for features not found
in standard tools like awk and lex. The syntax is a pair of
"question" exactly what is going on. That's psychology...
"(?#text)"
A comment. The text is ignored. If the "/x" modifier
enables whitespace formatting, a simple "#" will suffice.
Note that Perl closes the comment as soon as it sees a ")",
so there is no way to put a literal ")" in the comment.
"(?pimsx-imsx)"
One or more embedded pattern-match modifiers, to be turned on
(or turned off, if preceded by "-") for the remainder of the
pattern or the remainder of the enclosing pattern group (if
any). This is particularly useful for dynamic patterns, such
as those read in from a configuration file, taken from an
argument, or specified in a table somewhere. Consider the
case where some patterns want to be case sensitive and some
do not: The case insensitive ones merely need to include
"(?i)" at the front of the pattern. For example:
$pattern = "foobar";
if ( /$pattern/i ) { }
# more flexible:
$pattern = "(?i)foobar";
if ( /$pattern/ ) { }
These modifiers are restored at the end of the enclosing
group. For example,
( (?i) blah ) \s+ \1
will match "blah" in any case, some spaces, and an exact
(including the case!) repetition of the previous word,
assuming the "/x" modifier, and no "/i" modifier outside this
group.
Note that the "p" modifier is special in that it can only be
enabled, not disabled, and that its presence anywhere in a
pattern has a global effect. Thus "(?-p)" and "(?-p:...)" are
meaningless and will warn when executed under "use warnings".
"(?:pattern)"
"(?imsx-imsx:pattern)"
This is for clustering, not capturing; it groups
subexpressions like "()", but doesn't make backreferences as
"()" does. So
@fields = split(/\b(?:a|b|c)\b/)
is like
@fields = split(/\b(a|b|c)\b/)
"(?|pattern)"
This is the "branch reset" pattern, which has the special
property that the capture buffers are numbered from the same
starting point in each alternation branch. It is available
starting from perl 5.10.0.
Capture buffers are numbered from left to right, but inside
this construct the numbering is restarted for each branch.
The numbering within each branch will be as normal, and any
buffers following this construct will be numbered as though
the construct contained only one branch, that being the one
with the most capture buffers in it.
This construct will be useful when you want to capture one of
a number of alternative matches.
Consider the following pattern. The numbers underneath show
in which buffer the captured content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
Note: as of Perl 5.10.0, branch resets interfere with the
contents of the "%+" hash, that holds named captures.
Consider using "%-" instead.
Look-Around Assertions
Look-around assertions are zero width patterns which match a
specific pattern without including it in $&. Positive
assertions match when their subpattern matches, negative
assertions match when their subpattern fails. Look-behind
matches text up to the current match position, look-ahead
matches text following the current match position.
"(?=pattern)"
A zero-width positive look-ahead assertion. For example,
"/\w+(?=\t)/" matches a word followed by a tab, without
including the tab in $&.
"(?!pattern)"
A zero-width negative look-ahead assertion. For example
"/foo(?!bar)/" matches any occurrence of "foo" that isn't
followed by "bar". Note however that look-ahead and
look-behind are NOT the same thing. You cannot use this
for look-behind.
If you are looking for a "bar" that isn't preceded by a
"foo", "/(?!foo)bar/" will not do what you want. That's
because the "(?!foo)" is just saying that the next thing
cannot be "foo"--and it's not, it's a "bar", so "foobar"
will match. You would have to do something like
"/(?!foo)...bar/" for that. We say "like" because
tab, without including the tab in $&. Works only for
fixed-width look-behind.
There is a special form of this construct, called "\K",
which causes the regex engine to "keep" everything it had
matched prior to the "\K" and not include it in $&. This
effectively provides variable length look-behind. The use
of "\K" inside of another look-around assertion is
allowed, but the behaviour is currently not well defined.
For various reasons "\K" may be significantly more
efficient than the equivalent "(?<=...)" construct, and
it is especially useful in situations where you want to
efficiently remove something following something else in
a string. For instance
s/(foo)bar/$1/g;
can be rewritten as the much more efficient
s/foo\Kbar//g;
"(?<!pattern)"
A zero-width negative look-behind assertion. For example
"/(?<!bar)foo/" matches any occurrence of "foo" that does
not follow "bar". Works only for fixed-width look-
behind.
"(?'NAME'pattern)"
"(?<NAME>pattern)"
A named capture buffer. Identical in every respect to normal
capturing parentheses "()" but for the additional fact that
"%+" or "%-" may be used after a successful match to refer to
a named buffer. See "perlvar" for more details on the "%+"
and "%-" hashes.
If multiple distinct capture buffers have the same name then
the $+{NAME} will refer to the leftmost defined buffer in the
match.
The forms "(?'NAME'pattern)" and "(?<NAME>pattern)" are
equivalent.
NOTE: While the notation of this construct is the same as the
similar function in .NET regexes, the behavior is not. In
Perl the buffers are numbered sequentially regardless of
being named or not. Thus in the pattern
/(x)(?<foo>y)(z)/
$+{foo} will be the same as $2, and $3 will contain 'z'
instead of the opposite which is what a .NET regex hacker
might expect.
"\k'NAME'"
Named backreference. Similar to numeric backreferences,
except that the group is designated by name and not number.
If multiple groups have the same name then it refers to the
leftmost defined group in the current match.
It is an error to refer to a name not defined by a
"(?<NAME>)" earlier in the pattern.
Both forms are equivalent.
NOTE: In order to make things easier for programmers with
experience with the Python or PCRE regex engines, the pattern
"(?P=NAME)" may be used instead of "\k<NAME>".
"(?{ code })"
WARNING: This extended regular expression feature is
considered experimental, and may be changed without notice.
Code executed that has side effects may not perform
identically from version to version due to the effect of
future optimisations in the regex engine.
This zero-width assertion evaluates any embedded Perl code.
It always succeeds, and its "code" is not interpolated.
Currently, the rules to determine where the "code" ends are
somewhat convoluted.
This feature can be used together with the special variable
$^N to capture the results of submatches in variables without
having to keep track of the number of nested parentheses. For
example:
$_ = "The brown fox jumps over the lazy dog";
/the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
print "color = $color, animal = $animal\n";
Inside the "(?{...})" block, $_ refers to the string the
regular expression is matching against. You can also use
"pos()" to know what is the current position of matching
within this string.
The "code" is properly scoped in the following sense: If the
assertion is backtracked (compare "Backtracking"), all
changes introduced after "local"ization are undone, so that
$_ = 'a' x 8;
m<
(?{ $cnt = 0 }) # Initialize $cnt.
(
a
(?{
local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
})
)*
the special variable $^R. This happens immediately, so $^R
can be used from other "(?{ code })" assertions inside the
same regular expression.
The assignment to $^R above is properly localized, so the old
value of $^R is restored if the assertion is backtracked;
compare "Backtracking".
Due to an unfortunate implementation issue, the Perl code
contained in these blocks is treated as a compile time
closure that can have seemingly bizarre consequences when
used with lexically scoped variables inside of subroutines or
loops. There are various workarounds for this, including
simply using global variables instead. If you are using this
construct and strange results occur then check for the use of
lexically scoped variables.
For reasons of security, this construct is forbidden if the
regular expression involves run-time interpolation of
variables, unless the perilous "use re 'eval'" pragma has
been used (see re), or the variables contain results of
"qr//" operator (see "qr/STRING/imosx" in perlop).
This restriction is due to the wide-spread and remarkably
convenient custom of using run-time determined strings as
patterns. For example:
$re = <>;
chomp $re;
$string =~ /$re/;
Before Perl knew how to execute interpolated code within a
pattern, this operation was completely safe from a security
point of view, although it could raise an exception from an
illegal pattern. If you turn on the "use re 'eval'", though,
it is no longer secure, so you should only do so if you are
also using taint checking. Better yet, use the carefully
constrained evaluation within a Safe compartment. See
perlsec for details about both these mechanisms.
Because Perl's regex engine is currently not re-entrant,
interpolated code may not invoke the regex engine either
directly with "m//" or "s///"), or indirectly with functions
such as "split".
"(??{ code })"
WARNING: This extended regular expression feature is
considered experimental, and may be changed without notice.
Code executed that has side effects may not perform
identically from version to version due to the effect of
future optimisations in the regex engine.
This is a "postponed" regular subexpression. The "code" is
evaluated at run time, at the moment this subexpression may
The "code" is not interpolated. As before, the rules to
determine where the "code" ends are currently somewhat
convoluted.
The following pattern matches a parenthesized group:
$re = qr{
\(
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(??{ $re }) # Group with matching parens
)*
\)
}x;
See also "(?PARNO)" for a different, more efficient way to
accomplish the same task.
Because perl's regex engine is not currently re-entrant,
delayed code may not invoke the regex engine either directly
with "m//" or "s///"), or indirectly with functions such as
"split".
Recursing deeper than 50 times without consuming any input
string will result in a fatal error. The maximum depth is
compiled into perl, so changing it requires a custom build.
"(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
Similar to "(??{ code })" except it does not involve
compiling any code, instead it treats the contents of a
capture buffer as an independent pattern that must match at
the current position. Capture buffers contained by the
pattern will have the value as determined by the outermost
recursion.
PARNO is a sequence of digits (not starting with 0) whose
value reflects the paren-number of the capture buffer to
recurse to. "(?R)" recurses to the beginning of the whole
pattern. "(?0)" is an alternate syntax for "(?R)". If PARNO
is preceded by a plus or minus sign then it is assumed to be
relative, with negative numbers indicating preceding capture
buffers and positive ones following. Thus "(?-1)" refers to
the most recently declared buffer, and "(?+1)" indicates the
next buffer to be declared. Note that the counting for
relative recursion differs from that of relative
backreferences, in that with recursion unclosed buffers are
included.
The following pattern matches a function foo() which may
contain balanced parentheses as the argument.
$re = qr{ ( # paren group 1 (full function)
foo
}x;
If the pattern was used as follows
'foo(bar(baz)+baz(bop))'=~/$re/
and print "\$1 = $1\n",
"\$2 = $2\n",
"\$3 = $3\n";
the output produced should be the following:
$1 = foo(bar(baz)+baz(bop))
$2 = (bar(baz)+baz(bop))
$3 = bar(baz)+baz(bop)
If there is no corresponding capture buffer defined, then it
is a fatal error. Recursing deeper than 50 times without
consuming any input string will also result in a fatal error.
The maximum depth is compiled into perl, so changing it
requires a custom build.
The following shows how using negative indexing can make it
easier to embed recursive patterns inside of a "qr//"
construct for later use:
my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
if (/foo $parens \s+ + \s+ bar $parens/x) {
# do something here...
}
Note that this pattern does not behave the same way as the
equivalent PCRE or Python construct of the same form. In Perl
you can backtrack into a recursed group, in PCRE and Python
the recursed into group is treated as atomic. Also, modifiers
are resolved at compile time, so constructs like (?i:(?1)) or
(?:(?i)(?1)) do not affect how the sub-pattern will be
processed.
"(?&NAME)"
Recurse to a named subpattern. Identical to "(?PARNO)" except
that the parenthesis to recurse to is determined by name. If
multiple parentheses have the same name, then it recurses to
the leftmost.
It is an error to refer to a name that is not declared
somewhere in the pattern.
NOTE: In order to make things easier for programmers with
experience with the Python or PCRE regex engines the pattern
"(?P>NAME)" may be used instead of "(?&NAME)".
"(?(condition)yes-pattern|no-pattern)"
"(?(condition)yes-pattern)"
Conditional expression. "(condition)" should be either an
(1) (2) ...
Checks if the numbered capturing buffer has matched
something.
(<NAME>) ('NAME')
Checks if a buffer with the given name has matched
something.
(?{ CODE })
Treats the code block as the condition.
(R) Checks if the expression has been evaluated inside of
recursion.
(R1) (R2) ...
Checks if the expression has been evaluated while
executing directly inside of the n-th capture group. This
check is the regex equivalent of
if ((caller(0))[3] eq 'subname') { ... }
In other words, it does not check the full recursion
stack.
(R&NAME)
Similar to "(R1)", this predicate checks to see if we're
executing directly inside of the leftmost group with a
given name (this is the same logic used by "(?&NAME)" to
disambiguate). It does not check the full stack, but only
the name of the innermost active recursion.
(DEFINE)
In this case, the yes-pattern is never directly executed,
and no no-pattern is allowed. Similar in spirit to
"(?{0})" but more efficient. See below for details.
For example:
m{ ( \( )?
[^()]+
(?(1) \) )
}x
matches a chunk of non-parentheses, possibly included in
parentheses themselves.
A special form is the "(DEFINE)" predicate, which never
executes directly its yes-pattern, and does not allow a no-
pattern. This allows to define subpatterns which will be
executed only by using the recursion mechanism. This way,
you can define a set of regular expression rules that can be
bundled into any pattern you choose.
(?<NAME_PAT>....)
(?<ADRESS_PAT>....)
)/x
Note that capture buffers matched inside of recursion are not
accessible after the recursion returns, so the extra layer of
capturing buffers is necessary. Thus $+{NAME_PAT} would not
be defined even though $+{NAME} would be.
"(?>pattern)"
An "independent" subexpression, one which matches the
substring that a standalone "pattern" would match if anchored
at the given position, and it matches nothing other than this
substring. This construct is useful for optimizations of
what would otherwise be "eternal" matches, because it will
not backtrack (see "Backtracking"). It may also be useful in
places where the "grab all you can, and do not give anything
back" semantic is desirable.
For example: "^(?>a*)ab" will never match, since "(?>a*)"
(anchored at the beginning of string, as above) will match
all characters "a" at the beginning of string, leaving no "a"
for "ab" to match. In contrast, "a*ab" will match the same
as "a+b", since the match of the subgroup "a*" is influenced
by the following group "ab" (see "Backtracking"). In
particular, "a*" inside "a*ab" will match fewer characters
than a standalone "a*", since this makes the tail match.
An effect similar to "(?>pattern)" may be achieved by writing
"(?=(pattern))\1". This matches the same substring as a
standalone "a+", and the following "\1" eats the matched
string; it therefore makes a zero-length assertion into an
analogue of "(?>...)". (The difference between these two
constructs is that the second one uses a capturing group,
thus shifting ordinals of backreferences in the rest of a
regular expression.)
Consider this pattern:
m{ \(
(
[^()]+ # x+
|
\( [^()]* \)
)+
\)
}x
That will efficiently match a nonempty group with matching
parentheses two levels deep or less. However, if there is no
such group, it will take virtually forever on a long string.
That's because there are so many different ways to split a
long string into several substrings. This is what "(.+)+" is
doing, and "(.+)+" is similar to a subpattern of the above
\)
}x
which uses "(?>...)" matches exactly when the one above does
(verifying this yourself would be a productive exercise), but
finishes in a fourth the time when used on a similar string
with 1000000 "a"s. Be aware, however, that this pattern
currently triggers a warning message under the "use warnings"
pragma or -w switch saying it "matches null string many times
in regex".
On simple groups, such as the pattern "(?> [^()]+ )", a
comparable effect may be achieved by negative look-ahead, as
in "[^()]+ (?! [^()] )". This was only 4 times slower on a
string with 1000000 "a"s.
The "grab all you can, and do not give anything back"
semantic is desirable in many situations where on the first
sight a simple "()*" looks like the correct solution.
Suppose we parse text with comments being delimited by "#"
followed by some optional (horizontal) whitespace. Contrary
to its appearance, "#[ \t]*" is not the correct subexpression
to match the comment delimiter, because it may "give up" some
whitespace if the remainder of the pattern can be made to
match that way. The correct answer is either one of these:
(?>#[ \t]*)
#[ \t]*(?![ \t])
For example, to grab non-empty comments into $1, one should
use either one of these:
/ (?> \# [ \t]* ) ( .+ ) /x;
/ \# [ \t]* ( [^ \t] .* ) /x;
Which one you pick depends on which of these expressions
better reflects the above specification of comments.
In some literature this construct is called "atomic matching"
or "possessive matching".
Possessive quantifiers are equivalent to putting the item
they are applied to inside of one of these constructs. The
following equivalences apply:
Quantifier Form Bracketing Form
--------------- ---------------
PAT*+ (?>PAT*)
PAT++ (?>PAT+)
PAT?+ (?>PAT?)
PAT{min,max}+ (?>PAT{min,max})
Special Backtracking Control Verbs
WARNING: These patterns are experimental and subject to change or
On failure, the $REGERROR variable will be set to the ARG value of the
verb pattern, if the verb was involved in the failure of the match. If
the ARG part of the pattern was omitted, then $REGERROR will be set to
the name of the last "(*MARK:NAME)" pattern executed, or to TRUE if
there was none. Also, the $REGMARK variable will be set to FALSE.
On a successful match, the $REGERROR variable will be set to FALSE, and
the $REGMARK variable will be set to the name of the last
"(*MARK:NAME)" pattern executed. See the explanation for the
"(*MARK:NAME)" verb below for more details.
NOTE: $REGERROR and $REGMARK are not magic variables like $1 and most
other regex related variables. They are not local to a scope, nor
readonly, but instead are volatile package variables similar to
$AUTOLOAD. Use "local" to localize changes to them to a specific scope
if necessary.
If a pattern does not contain a special backtracking verb that allows
an argument, then $REGERROR and $REGMARK are not touched at all.
Verbs that take an argument
"(*PRUNE)" "(*PRUNE:NAME)"
This zero-width pattern prunes the backtracking tree at the
current point when backtracked into on failure. Consider the
pattern "A (*PRUNE) B", where A and B are complex patterns.
Until the "(*PRUNE)" verb is reached, A may backtrack as
necessary to match. Once it is reached, matching continues in
B, which may also backtrack as necessary; however, should B not
match, then no further backtracking will take place, and the
pattern will fail outright at the current starting position.
The following example counts all the possible matching strings
in a pattern (without actually matching any of them).
'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
which produces:
aaab
aaa
aa
a
aab
aa
a
ab
a
Count=9
If we add a "(*PRUNE)" before the count like the following
'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
ways to control backtracking. In some cases, the use of
"(*PRUNE)" can be replaced with a "(?>pattern)" with no
functional difference; however, "(*PRUNE)" can be used to
handle cases that cannot be expressed using a "(?>pattern)"
alone.
"(*SKIP)" "(*SKIP:NAME)"
This zero-width pattern is similar to "(*PRUNE)", except that
on failure it also signifies that whatever text that was
matched leading up to the "(*SKIP)" pattern being executed
cannot be part of any match of this pattern. This effectively
means that the regex engine "skips" forward to this position on
failure and tries to match again, (assuming that there is
sufficient room to match).
The name of the "(*SKIP:NAME)" pattern has special
significance. If a "(*MARK:NAME)" was encountered while
matching, then it is that position which is used as the "skip
point". If no "(*MARK)" of that name was encountered, then the
"(*SKIP)" operator has no effect. When used without a name the
"skip point" is where the match point was when executing the
(*SKIP) pattern.
Compare the following to the examples in "(*PRUNE)", note the
string is twice as long:
'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
outputs
aaab
aaab
Count=2
Once the 'aaab' at the start of the string has matched, and the
"(*SKIP)" executed, the next starting point will be where the
cursor was when the "(*SKIP)" was executed.
"(*MARK:NAME)" "(*:NAME)" "(*MARK:NAME)" "(*:NAME)"
This zero-width pattern can be used to mark the point reached
in a string when a certain part of the pattern has been
successfully matched. This mark may be given a name. A later
"(*SKIP)" pattern will then skip forward to that point if
backtracked into on failure. Any number of "(*MARK)" patterns
are allowed, and the NAME portion is optional and may be
duplicated.
In addition to interacting with the "(*SKIP)" pattern,
"(*MARK:NAME)" can be used to "label" a pattern branch, so that
after matching, the program can determine which branches of the
pattern were involved in the match.
When a match is successful, the $REGMARK variable will be set
recently executed "(*MARK:NAME)".
See "(*SKIP)" for more details.
As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".
"(*THEN)" "(*THEN:NAME)"
This is similar to the "cut group" operator "::" from Perl 6.
Like "(*PRUNE)", this verb always matches, and when backtracked
into on failure, it causes the regex engine to try the next
alternation in the innermost enclosing group (capturing or
otherwise).
Its name comes from the observation that this operation
combined with the alternation operator ("|") can be used to
create what is essentially a pattern-based if/then/else block:
( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
Note that if this operator is used and NOT inside of an
alternation then it acts exactly like the "(*PRUNE)" operator.
/ A (*PRUNE) B /
is the same as
/ A (*THEN) B /
but
/ ( A (*THEN) B | C (*THEN) D ) /
is not the same as
/ ( A (*PRUNE) B | C (*PRUNE) D ) /
as after matching the A but failing on the B the "(*THEN)" verb
will backtrack and try C; but the "(*PRUNE)" verb will simply
fail.
"(*COMMIT)"
This is the Perl 6 "commit pattern" "<commit>" or ":::". It's a
zero-width pattern similar to "(*SKIP)", except that when
backtracked into on failure it causes the match to fail
outright. No further attempts to find a valid match by
advancing the start pointer will occur again. For example,
'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
outputs
aaab
Count=1
It is probably useful only when combined with "(?{})" or
"(??{})".
"(*ACCEPT)"
WARNING: This feature is highly experimental. It is not
recommended for production code.
This pattern matches nothing and causes the end of successful
matching at the point at which the "(*ACCEPT)" pattern was
encountered, regardless of whether there is actually more to
match in the string. When inside of a nested pattern, such as
recursion, or in a subpattern dynamically generated via
"(??{})", only the innermost pattern is ended immediately.
If the "(*ACCEPT)" is inside of capturing buffers then the
buffers are marked as ended at the point at which the
"(*ACCEPT)" was encountered. For instance:
'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
will match, and $1 will be "AB" and $2 will be "B", $3 will not
be set. If another branch in the inner parentheses were
matched, such as in the string 'ACDE', then the "D" and "E"
would have to be matched as well.
Backtracking
NOTE: This section presents an abstract approximation of regular
expression behavior. For a more rigorous (and complicated) view of the
rules involved in selecting a match among possible alternatives, see
"Combining RE Pieces".
A fundamental feature of regular expression matching involves the
notion called backtracking, which is currently used (when needed) by
all regular non-possessive expression quantifiers, namely "*", "*?",
"+", "+?", "{n,m}", and "{n,m}?". Backtracking is often optimized
internally, but the general principle outlined here is valid.
For a regular expression to match, the entire regular expression must
match, not just part of it. So if the beginning of a pattern
containing a quantifier succeeds in a way that causes later parts in
the pattern to fail, the matching engine backs up and recalculates the
beginning part--that's why it's called backtracking.
Here is an example of backtracking: Let's say you want to find the
word following "foo" in the string "Food is on the foo table.":
$_ = "Food is on the foo table.";
if ( /\b(foo)\s+(\w+)/i ) {
print "$2 follows $1.\n";
}
When the match runs, the first part of the regular expression
("\b(foo)") finds a possible match right at the beginning of the
string, and loads up $1 with "Foo". However, as soon as the matching
if ( /foo(.*)bar/ ) {
print "got <$1>\n";
}
Which perhaps unexpectedly yields:
got <d is under the bar in the >
That's because ".*" was greedy, so you get everything between the first
"foo" and the last "bar". Here it's more effective to use minimal
matching to make sure you get the text between a "foo" and the first
"bar" thereafter.
if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
got <d is under the >
Here's another example. Let's say you'd like to match a number at the
end of a string, and you also want to keep the preceding part of the
match. So you write this:
$_ = "I have 2 numbers: 53147";
if ( /(.*)(\d*)/ ) { # Wrong!
print "Beginning is <$1>, number is <$2>.\n";
}
That won't work at all, because ".*" was greedy and gobbled up the
whole string. As "\d*" can match on an empty string the complete
regular expression matched successfully.
Beginning is <I have 2 numbers: 53147>, number is <>.
Here are some variants, most of which don't work:
$_ = "I have 2 numbers: 53147";
@pats = qw{
(.*)(\d*)
(.*)(\d+)
(.*?)(\d*)
(.*?)(\d+)
(.*)(\d+)$
(.*?)(\d+)$
(.*)\b(\d+)$
(.*\D)(\d+)$
};
for $pat (@pats) {
printf "%-12s ", $pat;
if ( /$pat/ ) {
print "<$1> <$2>\n";
} else {
print "FAIL\n";
}
}
regular expression is merely a set of assertions that gives a
definition of success. There may be 0, 1, or several different ways
that the definition might succeed against a particular string. And if
there are multiple ways it might succeed, you need to understand
backtracking to know which variety of success you will achieve.
When using look-ahead assertions and negations, this can all get even
trickier. Imagine you'd like to find a sequence of non-digits not
followed by "123". You might try to write that as
$_ = "ABC123";
if ( /^\D*(?!123)/ ) { # Wrong!
print "Yup, no 123 in $_\n";
}
But that isn't going to match; at least, not the way you're hoping. It
claims that there is no 123 in the string. Here's a clearer picture of
why that pattern matches, contrary to popular expectations:
$x = 'ABC123';
$y = 'ABC445';
print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
This prints
2: got ABC
3: got AB
4: got ABC
You might have expected test 3 to fail because it seems to a more
general purpose version of test 1. The important difference between
them is that test 3 contains a quantifier ("\D*") and so can use
backtracking, whereas test 1 will not. What's happening is that you've
asked "Is it true that at the start of $x, following 0 or more non-
digits, you have something that's not 123?" If the pattern matcher had
let "\D*" expand to "ABC", this would have caused the whole pattern to
fail.
The search engine will initially match "\D*" with "ABC". Then it will
try to match "(?!123" with "123", which fails. But because a
quantifier ("\D*") has been used in the regular expression, the search
engine can backtrack and retry the match differently in the hope of
matching the complete regular expression.
The pattern really, really wants to succeed, so it uses the standard
pattern back-off-and-retry and lets "\D*" expand to just "AB" this
time. Now there's indeed something following "AB" that is not "123".
It's "C123", which suffices.
In other words, the two zero-width assertions next to each other work
as though they're ANDed together, just as you'd use any built-in
assertions: "/^$/" matches only if you're at the beginning of the line
AND the end of the line simultaneously. The deeper underlying truth is
that juxtaposition in regular expressions always means AND, except when
you write an explicit OR using the vertical bar. "/ab/" means match
"a" AND (then) match "b", although the attempted matches are made at
different positions because "a" is not a zero-width assertion, but a
one-width assertion.
WARNING: Particularly complicated regular expressions can take
exponential time to solve because of the immense number of possible
ways they can use backtracking to try for a match. For example,
without internal optimizations done by the regular expression engine,
this will take a painfully long time to run:
'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
And if you used "*"'s in the internal groups instead of limiting them
to 0 through 5 matches, then it would take forever--or until you ran
out of stack space. Moreover, these internal optimizations are not
always applicable. For example, if you put "{0,5}" instead of "*" on
the external group, no current optimization is applicable, and the
match takes a long time to finish.
A powerful tool for optimizing such beasts is what is known as an
"independent group", which does not backtrack (see "(?>pattern)").
Note also that zero-length look-ahead/look-behind assertions will not
backtrack to make the tail match, since they are in "logical" context:
only whether they match is considered relevant. For an example where
side-effects of look-ahead might have influenced the following match,
see "(?>pattern)".
Version 8 Regular Expressions
In case you're not familiar with the "regular" Version 8 regex
routines, here are the pattern-matching rules not described above.
Any single character matches itself, unless it is a metacharacter with
a special meaning described here or above. You can cause characters
that normally function as metacharacters to be interpreted literally by
prefixing them with a "\" (e.g., "\." matches a ".", not any character;
"\\" matches a "\"). This escape mechanism is also required for the
character used as the pattern delimiter.
A series of characters matches that series of characters in the target
string, so the pattern "blurfl" would match "blurfl" in the target
string.
You can specify a character class, by enclosing a list of characters in
"[]", which will match any character from the list. If the first
character after the "[" is "^", the class matches any character not in
the list. Within a list, the "-" character specifies a range, so that
"a-z" represents all characters between "a" and "z", inclusive. If you
want either "-" or "]" itself to be a member of a class, put it at the
that begin from and end at either alphabetics of equal case ([a-e],
[A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt,
spell out the character sets in full.
Characters may be specified using a metacharacter syntax much like that
used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
"\f" a form feed, etc. More generally, \nnn, where nnn is a string of
octal digits, matches the character whose coded character set value is
nnn. Similarly, \xnn, where nn are hexadecimal digits, matches the
character whose numeric value is nn. The expression \cx matches the
character control-x. Finally, the "." metacharacter matches any
character except "\n" (unless you use "/s").
You can specify a series of alternatives for a pattern using "|" to
separate them, so that "fee|fie|foe" will match any of "fee", "fie", or
"foe" in the target string (as would "f(e|i|o)e"). The first
alternative includes everything from the last pattern delimiter ("(",
"[", or the beginning of the pattern) up to the first "|", and the last
alternative contains everything from the last "|" to the next pattern
delimiter. That's why it's common practice to include alternatives in
parentheses: to minimize confusion about where they start and end.
Alternatives are tried from left to right, so the first alternative
found for which the entire expression matches, is the one that is
chosen. This means that alternatives are not necessarily greedy. For
example: when matching "foo|foot" against "barefoot", only the "foo"
part will match, as that is the first alternative tried, and it
successfully matches the target string. (This might not seem important,
but it is important when you are capturing matched text using
parentheses.)
Also remember that "|" is interpreted as a literal within square
brackets, so if you write "[fee|fie|foe]" you're really only matching
"[feio|]".
Within a pattern, you may designate subpatterns for later reference by
enclosing them in parentheses, and you may refer back to the nth
subpattern later in the pattern using the metacharacter \n.
Subpatterns are numbered based on the left to right order of their
opening parenthesis. A backreference matches whatever actually matched
the subpattern in the string being examined, not the rules for that
subpattern. Therefore, "(0|0x)\d*\s\1\d*" will match "0x1234 0x4321",
but not "0x1234 01234", because subpattern 1 matched "0x", even though
the rule "0|0x" could potentially match the leading 0 in the second
number.
Warning on \1 Instead of $1
Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered for the RHS of a substitute to avoid shocking the
sed addicts, but it's a dirty habit to get into. That's because in
PerlThink, the righthand side of an "s///" is a double-quoted string.
with "${1}000". The operation of interpolation should not be confused
with the operation of matching a backreference. Certainly they mean
two different things on the left side of the "s///".
Repeated Patterns Matching a Zero-length Substring
WARNING: Difficult material (and prose) ahead. This section needs a
rewrite.
Regular expressions provide a terse and powerful programming language.
As with most other power tools, power comes together with the ability
to wreak havoc.
A common abuse of this power stems from the ability to make infinite
loops using regular expressions, with something as innocuous as:
'foo' =~ m{ ( o? )* }x;
The "o?" matches at the beginning of 'foo', and since the position in
the string is not moved by the match, "o?" would match again and again
because of the "*" quantifier. Another common way to create a similar
cycle is with the looping modifier "//g":
@matches = ( 'foo' =~ m{ o? }xg );
or
print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
or the loop implied by split().
However, long experience has shown that many programming tasks may be
significantly simplified by using repeated subexpressions that may
match zero-length substrings. Here's a simple example being:
@chars = split //, $string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
Thus Perl allows such constructs, by forcefully breaking the infinite
loop. The rules for this are different for lower-level loops given by
the greedy quantifiers "*+{}", and for higher-level ones like the "/g"
modifier or split() operator.
The lower-level loops are interrupted (that is, the loop is broken)
when Perl detects that a repeated expression matched a zero-length
substring. Thus
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
is made equivalent to
m{ (?: NON_ZERO_LENGTH )*
|
(?: ZERO_LENGTH )?
}x;
results in "<> |
