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 "$" from matching the start or end of the string to matching the start or end of any line anywhere within the string.
s: Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which normally it would not match.; Used together, as /ms, they let the "." match any character whatsoever, while still allowing "^" and "$" 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 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.
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 the perlretut manpage 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 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

Metacharacters

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 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.) The "*" quantifier is equivalent to {0,}, the "+" quantifier to {1,}, and the "?" quantifier 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

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)
    \cK         control char          (example: VT)
    \N{name}    named Unicode character
    \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/.

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 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.

\R will atomically match a linebreak, including the network line-ending "\x0D\x0A". Specifically, is exactly equivalent to

  (?>\x0D\x0A?|[\x0A-\x0C\x85\x{2028}\x{2029}])

Note: \R has no special meaning inside of a character class; use \v instead (vertical whitespace).

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 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}
    [[:^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.