Note: The following information will be moved into the user's guide.

Perl5 regular expressions

Here we summarize the syntax of Perl5.003 regular expressions, all of which is supported by the Perl5 classes in this package. However, for a definitive reference, you should consult the perlre man page that accompanies the Perl5 distribution and also the book Programming Perl, 2nd Edition from O'Reilly & Associates. We are working toward implementing the features added after Perl5.003 up to and including Perl 5.6. Please remember, we only guarantee support for Perl5.003 expressions in version 2.0.

• Alternatives separated by |
• Quantified atoms

  {n,m}

  Match at least n but not more than m times.

  {n,}

  Match at least n times.

  {n}

  Match exactly n times.

  *

  Match 0 or more times.

  +

  Match 1 or more times.

  ?

  Match 0 or 1 times.

• Atoms
  • regular expression within parentheses
  • a . matches everything except \n
  • a ^ is a null token matching the beginning of a string or line (i.e., the position right after a newline or right before the beginning of a string)
  • a $ is a null token matching the end of a string or line (i.e., the position right before a newline or right after the end of a string)
  • Character classes (e.g., [abcd]) and ranges (e.g. [a-z])
    • Special backslashed characters work within a character class (except for backreferences and boundaries).
    • \b is backspace inside a character class
  • Special backslashed characters

    \b

    null token matching a word boundary (\w on one side and \W on the other)

    \B

    null token matching a boundary that isn't a word boundary

    \A

    Match only at beginning of string

    \Z

    Match only at end of string (or before newline at the end)

    \n

    newline

    \r

    carriage return

    \t

    tab

    \f

    formfeed

    \d

    digit [0-9]

    \D

    non-digit [^0-9]

    \w

    word character [0-9a-z_A-Z]

    \W

    a non-word character [^0-9a-z_A-Z]

    \s

    a whitespace character [ \t\n\r\f]

    \S

    a non-whitespace character [^ \t\n\r\f]

    \xnn

    hexadecimal representation of character

    \cD

    matches the corresponding control character

    \nn or \nnn

    octal representation of character unless a backreference. a

    \1, \2, \3, etc.

    match whatever the first, second, third, etc. parenthesized group matched. This is called a backreference. If there is no corresponding group, the number is interpreted as an octal representation of a character.

    \0

    matches null character

    Any other backslashed character matches itself

• Expressions within parentheses are matched as subpattern groups and saved for use by certain methods.

By default, a quantified subpattern is greedy . In other words it matches as many times as possible without causing the rest of the pattern not to match. To change the quantifiers to match the minimum number of times possible, without causing the rest of the pattern not to match, you may use a "?" right after the quantifier.

*?

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

Perl5 extended regular expressions are fully supported.

(?#text)

An embedded comment causing text to be ignored.

(?:regexp)

Groups things like "()" but doesn't cause the group match to be saved.

(?=regexp)

A zero-width positive lookahead assertion. For example, \w+(?=\s) matches a word followed by whitespace, without including whitespace in the MatchResult.

(?!regexp)

A zero-width negative lookahead assertion. For example foo(?!bar) matches any occurrence of "foo" that isn't followed by "bar". Remember that this is a zero-width assertion, which means that a(?!b)d will match ad because a is followed by a character that is not b (the d) and a d follows the zero-width assertion.

(?imsx)

One or more embedded pattern-match modifiers. i enables case insensitivity, m enables multiline treatment of the input, s enables single line treatment of the input, and x enables extended whitespace comments.