find.info: awk regular expression syntax

Regular Expressions

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7.5.2 `awk' regular expression syntax
-------------------------------------

The character `.' matches any single character except the null
character.

`+'
     indicates that the regular expression should match one or more
     occurrences of the previous atom or regexp.

`?'
     indicates that the regular expression should match zero or one
     occurrence of the previous atom or regexp.

`\+'
     matches a `+'

`\?'
     matches a `?'.

   Bracket expressions are used to match ranges of characters.  Bracket
expressions where the range is backward, for example `[z-a]', are
invalid.  Within square brackets, `\' can be used to quote the
following character.  Character classes are not supported, so for
example you would need to use `[0-9]' instead of `[[:digit:]]'.

   GNU extensions are not supported and so `\w', `\W', `\<', `\>',
`\b', `\B', `\`', and `\'' match `w', `W', `<', `>', `b', `B', ``', and
`'' respectively.

   Grouping is performed with parentheses `()'.  An unmatched `)'
matches just itself.  A backslash followed by a digit matches that
digit.

   The alternation operator is `|'.

   The characters `^' and `$' always represent the beginning and end of
a string respectively, except within square brackets.  Within brackets,
`^' can be used to invert the membership of the character class being
specified.

   `*', `+' and `?' are special at any point in a regular expression
except:
  1. At the beginning of a regular expression

  2. After an open-group, signified by `('

  3. After the alternation operator `|'


   The longest possible match is returned; this applies to the regular
expression as a whole and (subject to this constraint) to
subexpressions within groups.