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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 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":
*? 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)
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 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$unquoted/
Perl defines a consistent extension syntax for regular
expressions. The syntax is a pair of parentheses with a question
mark as the first thing within the parentheses (this was a
syntax error in older versions of Perl). The character after the
question mark gives the function of the extension. Several
extensions are already supported:
(?#text) A comment. The text is ignored. If the `/x' switch is used
to enable whitespace formatting, a simple `#' will
suffice.
(?:regexp)
This groups things like "()" but doesn't make
backreferences like "()" does. So
split(/\b(?:a|b|c)\b/)
is like
split(/\b(a|b|c)\b/)
but doesn't spit out extra fields.
(?=regexp)
A zero-width positive lookahead assertion. For
example, `/\w+(?=\t)/' matches a word followed by a
tab, without including the tab in `$&'.
(?!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.
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.
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";
}
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 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, \x*nn*, where *nn* are
hexadecimal digits, matches the character whose ASCII value is
*nn*. The expression \c*x* matches the ASCII 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').
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 *n*th 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.