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gawk: Regexp Summary

 3.9 Summary
    * Regular expressions describe sets of strings to be matched.  In
      'awk', regular expression constants are written enclosed between
      slashes: '/'...'/'.
    * Regexp constants may be used standalone in patterns and in
      conditional expressions, or as part of matching expressions using
      the '~' and '!~' operators.
    * Escape sequences let you represent nonprintable characters and also
      let you represent regexp metacharacters as literal characters to be
    * Regexp operators provide grouping, alternation, and repetition.
    * Bracket expressions give you a shorthand for specifying sets of
      characters that can match at a particular point in a regexp.
      Within bracket expressions, POSIX character classes let you specify
      certain groups of characters in a locale-independent fashion.
    * Regular expressions match the leftmost longest text in the string
      being matched.  This matters for cases where you need to know the
      extent of the match, such as for text substitution and when the
      record separator is a regexp.
    * Matching expressions may use dynamic regexps (i.e., string values
      treated as regular expressions).
    * 'gawk''s 'IGNORECASE' variable lets you control the case
      sensitivity of regexp matching.  In other 'awk' versions, use
      'tolower()' or 'toupper()'.
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