PERLRE(1) PERLRE(1)
NAME
perlre - Perl regular expressions
DESCRIPTION
This page describes the syntax of regular expressions in Perl. For a
description of how to use regular expressions in matching operations,
plus various examples of the same, see m// and s/// in the perlop
manpage.
The matching operations can have various modifiers. The modifiers which
relate to the interpretation of the regular expression inside are listed
below. For the modifiers that alter the behaviour of the operation, see
the section on m// in the perlop manpage and the section on s// in the
perlop manpage.
i Do case-insensitive pattern matching.
If use locale is in effect, the case map is taken from the current
locale. See the perllocale manpage.
m Treat string as multiple lines. That is, change "^" and "$" from
matching at only the very start or end of the string to 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 it normally would not
match.
x Extend your pattern's legibility by permitting whitespace and
comments.
These are usually written as "the /x modifier", even though the delimiter
in question might not actually be a slash. In fact, any of these
modifiers may also be embedded within the regular expression itself using
the new (?...) construct. See below.
The /x modifier itself needs a little more explanation. It tells the
regular expression parser to ignore whitespace that is neither
backslashed nor within a character class. You can use this to break up
your regular expression into (slightly) more readable parts. The #
character is also treated as a metacharacter introducing a comment, just
as in ordinary Perl code. This also means that if you want real
whitespace or # characters in the pattern that you'll have to either
escape them or encode them using octal or hex escapes. Taken together,
these features go a long way towards making Perl's regular expressions
more readable. See the C comment deletion code in the perlop manpage.
Regular Expressions
The patterns used in pattern matching are regular expressions such as
those supplied in the Version 8 regexp routines. (In fact, the routines
are derived (distantly) from Henry Spencer's freely redistributable
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reimplementation of the V8 routines.) See the section on 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 at only the
beginning of the string, the "$" character at only the end (or before the
newline at the end) and Perl does certain optimizations with the
assumption that the string contains only one line. Embedded newlines
will not be matched by "^" or "$". You may, however, wish to treat a
string as a multi-line buffer, such that the "^" will match after any
newline within the string, 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 is now deprecated.)
To facilitate 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. The /s modifier
also overrides the setting of $*, in case you have some (badly behaved)
older code that sets it in another module.
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.) The "*" modifier is equivalent to {0,}, the "+"
modifier to {1,}, and the "?" modifier to {0,1}. n and m are limited to
integral values less than 65536.
By default, a quantified subpattern is "greedy", that is, it will match
as many times as possible (given a particular starting location) while
still allowing the rest of the pattern to match. If you want it to match
the minimum number of times possible, follow the quantifier with a "?".
Note that the meanings don't change, just the "greediness":
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*? Match 0 or more times
+? Match 1 or more times
?? Match 0 or 1 time
{n}? Match exactly n times
{n,}? Match at least n times
{n,m}? Match at least n but not more than m times
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 (think of a PDP-11)
\x1B hex char
\c[ control char
\l lowercase next char (think vi)
\u uppercase next char (think vi)
\L lowercase till \E (think vi)
\U uppercase till \E (think vi)
\E end case modification (think vi)
\Q quote (disable) regexp metacharacters till \E
If use locale is in effect, the case map used by \l, \L, \u and <\U> is
taken from the current locale. See the perllocale manpage.
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
Note that \w matches a single alphanumeric character, not a whole word.
To match a word you'd need to say \w+. If use locale is in effect, the
list of alphabetic characters generated by \w is taken from the current
locale. See the perllocale manpage. You may use \w, \W, \s, \S, \d, and
\D within character classes (though not as either end of a range).
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match a non-(word boundary)
\A Match at only beginning of string
\Z Match at only end of string (or before newline at the end)
\G Match only where previous m//g left off (works only with /g)
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A word boundary (\b) is defined as 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.) The \A and \Z are just like "^"
and "$" except that they won't match multiple times when the /m modifier
is used, while "^" and "$" will match at every internal line boundary.
To match the actual end of the string, not ignoring newline, you can use
\Z(?!\n). The \G assertion can be used to chain global matches (using
m//g), as described in the section on Regexp Quote-Like Operators in the
perlop manpage.
It is also useful when writing lex-like scanners, when you have several
regexps which you want to match against consequent substrings of your
string, see the previous reference. The actual location where \G will
match can also be influenced by using pos() as an lvalue. See the pos
entry in the perlfunc manpage.
When the bracketing construct ( ... ) is used, \<digit> matches the
digit'th substring. Outside of the pattern, always use "$" instead of
"\" in front of the digit. (While the \<digit> notation can on rare
occasion work outside the current pattern, this should not be relied
upon. See the WARNING below.) The scope of $<digit> (and $`, $&, and $')
extends to the end of the enclosing BLOCK or eval string, or to the next
successful pattern match, whichever comes first. If you want to use
parentheses to delimit a subpattern (e.g., a set of alternatives) without
saving it as a subpattern, follow the ( with a ?:.
You may have as many parentheses as you wish. If you have more than 9
substrings, the variables $10, $11, ... refer to the corresponding
substring. Within the pattern, \10, \11, etc. refer back to substrings
if there have been at least that many left parentheses before the
backreference. Otherwise (for backward compatibility) \10 is the same as
\010, a backspace, and \11 the same as \011, a tab. And so on. (\1
through \9 are always backreferences.)
$+ returns whatever the last bracket match matched. $& returns the
entire matched string. ($0 used to return the same thing, but not any
more.) $` returns everything before the matched string. $' returns
everything after the matched string. Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
if (/Time: (..):(..):(..)/) {
$hours = $1;
$minutes = $2;
$seconds = $3;
}
Once perl sees that you need one of $&, $` or $' anywhere in the program,
it has to provide them on each and every pattern match. This can slow
your program down. The same mechanism that handles these provides for
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the use of $1, $2, etc., so you pay the same price for each regexp that
contains capturing parentheses. But if you never use $&, etc., in your
script, then regexps without capturing parentheses won't 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.
You will note that all 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 the non-alphanumeric characters:
$pattern =~ s/(\W)/\\$1/g;
Now it is much more common to see either the quotemeta() function or the
\Q escape sequence used to disable the metacharacters special meanings
like this:
/$unquoted\Q$quoted\E$unquoted/
Perl defines a consistent extension syntax for regular expressions. The
syntax is a pair of parentheses with a question mark as the first thing
within the parentheses (this was a syntax error in older versions of
Perl). The character after the question mark gives the function of the
extension. Several extensions are already supported:
(?#text) A comment. The text is ignored. If the /x switch is used to
enable whitespace formatting, a simple # will suffice.
(?:regexp)
This groups things like "()" but doesn't make backreferences
like "()" does. So
split(/\b(?:a|b|c)\b/)
is like
split(/\b(a|b|c)\b/)
but doesn't spit out extra fields.
(?=regexp)
A zero-width positive lookahead assertion. For example,
/\w+(?=\t)/ matches a word followed by a tab, without including
the tab in $&.
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(?!regexp)
A zero-width negative lookahead assertion. For example
/foo(?!bar)/ matches any occurrence of "foo" that isn't
followed by "bar". Note however that lookahead and lookbehind
are NOT the same thing. You cannot use this for lookbehind:
/(?!foo)bar/ will not find an occurrence of "bar" that is
preceded by something which is not "foo". 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 there's the case of your "bar" not having three
characters before it. You could cover that this way:
/(?:(?!foo)...|^..?)bar/. Sometimes it's still easier just to
say:
if (/foo/ && $` =~ /bar$/)
(?imsx) One or more embedded pattern-match modifiers. This is
particularly useful for patterns that are specified in a table
somewhere, some of which want to be case sensitive, and some of
which don't. The case insensitive ones need to include merely
(?i) at the front of the pattern. For example:
$pattern = "foobar";
if ( /$pattern/i )
# more flexible:
$pattern = "(?i)foobar";
if ( /$pattern/ )
The specific choice of question mark for this and the new minimal
matching construct was because 1) question mark is pretty rare in older
regular expressions, and 2) whenever you see one, you should stop and
"question" exactly what is going on. That's psychology...
Backtracking
A fundamental feature of regular expression matching involves the notion
called backtracking. which is used (when needed) by all regular
expression quantifiers, namely *, *?, +, +?, {n,m}, and {n,m}?.
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.
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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 engine sees that there's
no whitespace following the "Foo" that it had saved in $1, it realizes
its mistake and starts over again one character after where it had the
tentative match. This time it goes all the way until the next occurrence
of "foo". The complete regular expression matches this time, and you get
the expected output of "table follows foo."
Sometimes minimal matching can help a lot. Imagine you'd like to match
everything between "foo" and "bar". Initially, you write something like
this:
$_ = "The food is under the bar in the barn.";
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". In this case, 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 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.
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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> <$2>\n";
} else {
print "FAIL\n";
}
}
That will print out:
(.*)(\d*) <I have 2 numbers: 53147> <>
(.*)(\d+) <I have 2 numbers: 5314> <7>
(.*?)(\d*) <> <>
(.*?)(\d+) <I have > <2>
(.*)(\d+)$ <I have 2 numbers: 5314> <7>
(.*?)(\d+)$ <I have 2 numbers: > <53147>
(.*)\b(\d+)$ <I have 2 numbers: > <53147>
(.*\D)(\d+)$ <I have 2 numbers: > <53147>
As you see, this can be a bit tricky. It's important to realize that a
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 lookahead assertions and negations, this can all get even
tricker. 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";
}
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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 it 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, of course, 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.
Well now, the pattern really, really wants to succeed, so it uses the
standard regexp 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 in fact "C123", which suffices.
We can deal with this by using both an assertion and a negation. We'll
say that the first part in $1 must be followed by a digit, and in fact,
it must also be followed by something that's not "123". Remember that
the lookaheads are zero-width expressions--they only look, but don't
consume any of the string in their match. So rewriting this way produces
what you'd expect; that is, case 5 will fail, but case 6 succeeds:
print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
6: got ABC
In other words, the two zero-width assertions next to each other work
like they're ANDed together, just as you'd use any builtin assertions:
/^$/ matches only if you're at the beginning of the line AND the end of
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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.
One warning: particularly complicated regular expressions can take
exponential time to solve due to the immense number of possible ways they
can use backtracking to try match. For example this will take a very
long time to run
/((a{0,5}){0,5}){0,5}/
And if you used *'s instead of limiting it to 0 through 5 matches, then
it would take literally forever--or until you ran out of stack space.
Version 8 Regular Expressions
In case you're not familiar with the "regular" Version 8 regexp 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 which
normally function as metacharacters to be interpreted literally by
prefixing them with a "\" (e.g., "\." matches a ".", not any character;
"\\" matches a "\"). 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 one of the characters in the list. If the first
character after the "[" is "^", the class matches any character not in
the list. Within a list, the "-" character is used to specify a range,
so that a-z represents all the characters between "a" and "z", inclusive.
If you want "-" itself to be a member of a class, put it at the start or
end of the list, or escape it with a backslash. (The following all
specify the same class of three characters: [-az], [az-], and [a\-z].
All are different from [a-z], which specifies a class containing twenty-
six characters.)
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 ASCII value is nnn. Similarly,
\xnn, where nn are hexadecimal digits, matches the character whose ASCII
value is nn. The expression \cx matches the ASCII character control-x.
Finally, the "." metacharacter matches any character except "\n" (unless
you use /s).
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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). Note that 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. For this reason, it's common practice to include alternatives
in parentheses, to minimize confusion about where they start and end.
Note however that "|" is interpreted as a literal with 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. Note that 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 actually matched "0x", even
though the rule 0|0x could potentially match the leading 0 in the second
number.
WARNING on \1 vs $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 a s/// is a double-quoted string. \1 in
the usual double-quoted string means a control-A. The customary Unix
meaning of \1 is kludged in for s///. However, if you get into the habit
of doing that, you get yourself into trouble if you then add an /e
modifier.
s/(\d+)/ \1 + 1 /eg;
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying \{1}000, whereas you can fix it
with ${1}000. Basically, 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///.
SEE ALSO
"Mastering Regular Expressions" (see the perlbook manpage) by Jeffrey
Friedl.
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