If you haven't used regular expressions before, a quick-start introduction is available in the perlrequick manpage, and a longer tutorial introduction is available in the perlretut manpage.

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 the perlop manpage.

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 the perlop manpage and Gory details of parsing quoted constructs in the perlop manpage.

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.; The /s and /m modifiers both override the $* setting. That is, no matter what $* contains, /s without /m will force "^" to match only at the beginning of the string and "$" to match only at the end (or just before a newline at the end) of the string. 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 the perllocale manpage.
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.

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 using the (?...) 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 (outside a character class, where they are unaffected by /x), then you'll either have to escape them (using backslashes or \Q...\E) or encode them using octal or hex escapes. Taken together, these features go a long way towards making 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 the perlop manpage. Also note that anything inside a \Q...\E stays unaffected by /x.

Regular Expressions

The patterns used in Perl pattern matching evolved from the ones 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 is now deprecated.)

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. 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. In particular, the lower bound is not optional.) 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 a preset limit defined when perl is built. This is usually 32766 on the most common platforms. The actual limit can be seen in the error message generated by code such as this:

    $_ **= $_ , / {$_} / for 2 .. 42;

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, 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

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}    wide hex char         (example: Unicode SMILEY)
    \cK         control char          (example: VT)
    \N{name}    named 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) pattern 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. For documentation of \N{name}, see the charnames manpage.

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/.

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.

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 the perllocale manpage. 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 the perlunicode manpage for more details about \pP, \PP, \X and the possibility of defining your own \p and \P properties, and the perluniintro manpage about Unicode in general.

The POSIX character class syntax

    [:class:]

is also available. Note that the [ and ] brackets are literal; they must always be used within a character class expression.

    # 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
    punct       IsPunct
    space       IsSpace
                IsSpacePerl    \s
    upper       IsUpper
    word        IsWord
    xdigit      IsXDigit

For example [[:lower:]] and \p{IsLower} are equivalent.

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").

The assumedly non-obviously 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}
    [[:^space:]]    \S         \P{IsSpace}
    [[:^word:]]     \W         \P{IsWord}

Perl respects the POSIX standard in that POSIX character classes are only supported within a character class. The POSIX character classes [.cc.] and [=cc=] are recognized but not supported and trying to use them will cause an error.

Perl defines the following zero-width assertions:

    \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 the perlop manpage. 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 the perlfunc manpage. Currently \G is only fully supported when anchored to the start of the pattern; while it is permitted to use it elsewhere, as in /(?<=\G..)./g, some such uses (/.\G/g, for example) currently cause problems, and it is recommended that you avoid such usage for now.

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.

There is no limit to the number of captured substrings that you may use. However Perl also uses \10, \11, etc. as aliases for \010, \011, etc. (Recall that 0 means octal, so \011 is the character at number 9 in your coded character set; which would be the 10th character, a horizontal tab under ASCII.) Perl resolves this ambiguity by interpreting \10 as a backreference only if at least 10 left parentheses have opened before it. Likewise \11 is a backreference only if at least 11 left parentheses have opened before it. And so on. \1 through \9 are always interpreted as backreferences.

Examples:

    s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

     if (/(.)\1/) {                 # find first doubled char
         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 match. $+ returns whatever the last bracket match matched. $& returns the entire matched string. (At one point $0 did also, but now it returns the name of the program.) $` returns everything before the matched string. $' returns everything after the matched string. And $^N contains whatever was matched by the most-recently closed group (submatch). $^N can be used in extended patterns (see below), for example to assign a submatch to a variable.

The numbered match variables ($1, $2, $3, etc.) and the related punctuation set ($+, $&, $`, $', and $^N) are all dynamically scoped until the end of the enclosing block or until the next successful match, whichever comes first. (See Compound Statements in the perlsyn manpage.)

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.

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 the perlop manpage.

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 parentheses with a question mark as the first thing within the parentheses. The character after the question mark indicates the extension.

The stability of these extensions varies widely. Some have been part of the core language for many years. Others are experimental and may change without warning or be completely removed. Check the documentation on an individual feature to verify its current status.

A question mark was chosen for this and for the minimal-matching construct because 1) question marks are rare in older regular expressions, and 2) whenever you see one, you should stop and "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.

(?imsx-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.

(?: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/)

but doesn't spit out extra fields. It's also cheaper not to capture characters if you don't need to.

Any letters between ? and : act as flags modifiers as with (?imsx-imsx). For example,

    /(?s-i:more.*than).*million/i

is equivalent to the more verbose

    /(?:(?s-i)more.*than).*million/i

(?=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 there's the case of your "bar" not having three characters before it. You could cover that this way: /(?:(?!foo)...|^.{0,2})bar/. Sometimes it's still easier just to say:

    if (/bar/ && $` !~ /foo$/)

For look-behind see below.

(?<=pattern)

A zero-width positive look-behind assertion. For example, /(?<=\t)\w+/ matches a word that follows a tab, without including the tab in $&. Works only for fixed-width look-behind.

(?<!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.

(?{ code })

WARNING: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice.

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.