( Regular Expressions

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 Overview of Regular Expression Syntax
    To know how to use `sed', people should understand regular
 expressions ("regexp" for short).  A regular expression is a pattern
 that is matched against a subject string from left to right.  Most
 characters are "ordinary": they stand for themselves in a pattern, and
 match the corresponding characters in the subject.  As a trivial
 example, the pattern
           The quick brown fox
 matches a portion of a subject string that is identical to itself.  The
 power of regular expressions comes from the ability to include
 alternatives and repetitions in the pattern.  These are encoded in the
 pattern by the use of "special characters", which do not stand for
 themselves but instead are interpreted in some special way.  Here is a
 brief description of regular expression syntax as used in `sed'.
      A single ordinary character matches itself.
      Matches a sequence of zero or more instances of matches for the
      preceding regular expression, which must be an ordinary character,
      a special character preceded by `\', a `.', a grouped regexp (see
      below), or a bracket expression.  As a GNU extension, a postfixed
      regular expression can also be followed by `*'; for example, `a**'
      is equivalent to `a*'.  POSIX 1003.1-2001 says that `*' stands for
      itself when it appears at the start of a regular expression or
      subexpression, but many nonGNU implementations do not support this
      and portable scripts should instead use `\*' in these contexts.
      As `*', but matches one or more.  It is a GNU extension.
      As `*', but only matches zero or one.  It is a GNU extension.
      As `*', but matches exactly I sequences (I is a decimal integer;
      for portability, keep it between 0 and 255 inclusive).
      Matches between I and J, inclusive, sequences.
      Matches more than or equal to I sequences.
      Groups the inner REGEXP as a whole, this is used to:
         * Apply postfix operators, like `\(abcd\)*': this will search
           for zero or more whole sequences of `abcd', while `abcd*'
           would search for `abc' followed by zero or more occurrences
           of `d'.  Note that support for `\(abcd\)*' is required by
           POSIX 1003.1-2001, but many non-GNU implementations do not
           support it and hence it is not universally portable.
         * Use back references (see below).
      Matches any character, including newline.
      Matches the null string at beginning of line, i.e. what appears
      after the circumflex must appear at the beginning of line.
      `^#include' will match only lines where `#include' is the first
      thing on line--if there are spaces before, for example, the match
      fails.  `^' acts as a special character only at the beginning of
      the regular expression or subexpression (that is, after `\(' or
      `\|').  Portable scripts should avoid `^' at the beginning of a
      subexpression, though, as POSIX allows implementations that treat
      `^' as an ordinary character in that context.
      It is the same as `^', but refers to end of line.  `$' also acts
      as a special character only at the end of the regular expression
      or subexpression (that is, before `\)' or `\|'), and its use at
      the end of a subexpression is not portable.
      Matches any single character in LIST: for example, `[aeiou]'
      matches all vowels.  A list may include sequences like
      `CHAR1-CHAR2', which matches any character between (inclusive)
      CHAR1 and CHAR2.
      A leading `^' reverses the meaning of LIST, so that it matches any
      single character _not_ in LIST.  To include `]' in the list, make
      it the first character (after the `^' if needed), to include `-'
      in the list, make it the first or last; to include `^' put it
      after the first character.
      The characters `$', `*', `.', `[', and `\' are normally not
      special within LIST.  For example, `[\*]' matches either `\' or
      `*', because the `\' is not special here.  However, strings like
      `[.ch.]', `[=a=]', and `[:space:]' are special within LIST and
      represent collating symbols, equivalence classes, and character
      classes, respectively, and `[' is therefore special within LIST
      when it is followed by `.', `=', or `:'.  Also, when not in
      `POSIXLY_CORRECT' mode, special escapes like `\n' and `\t' are
      recognized within LIST.   Escapes.
      Matches either REGEXP1 or REGEXP2.  Use parentheses to use complex
      alternative regular expressions.  The matching process tries each
      alternative in turn, from left to right, and the first one that
      succeeds is used.  It is a GNU extension.
      Matches the concatenation of REGEXP1 and REGEXP2.  Concatenation
      binds more tightly than `\|', `^', and `$', but less tightly than
      the other regular expression operators.
      Matches the DIGIT-th `\(...\)' parenthesized subexpression in the
      regular expression.  This is called a "back reference".
      Subexpressions are implicity numbered by counting occurrences of
      `\(' left-to-right.
      Matches the newline character.
      Matches CHAR, where CHAR is one of `$', `*', `.', `[', `\', or `^'.
      Note that the only C-like backslash sequences that you can
      portably assume to be interpreted are `\n' and `\\'; in particular
      `\t' is not portable, and matches a `t' under most implementations
      of `sed', rather than a tab character.
    Note that the regular expression matcher is greedy, i.e., matches
 are attempted from left to right and, if two or more matches are
 possible starting at the same character, it selects the longest.
      Matches `abcdef'.
      Matches zero or more `a's followed by a single `b'.  For example,
      `b' or `aaaaab'.
      Matches `b' or `ab'.
      Matches one or more `a's followed by one or more `b's: `ab' is the
      shortest possible match, but other examples are `aaaab' or
      `abbbbb' or `aaaaaabbbbbbb'.
      These two both match all the characters in a string; however, the
      first matches every string (including the empty string), while the
      second matches only strings containing at least one character.
      his matches a string starting with `main', followed by an opening
      and closing parenthesis.  The `n', `(' and `)' need not be
      This matches a string beginning with `#'.
      This matches a string ending with a single backslash.  The regexp
      contains two backslashes for escaping.
      Instead, this matches a string consisting of a single dollar sign,
      because it is escaped.
      In the C locale, this matches any ASCII letters or digits.
 `[^ tab]\+'
      (Here `tab' stands for a single tab character.)  This matches a
      string of one or more characters, none of which is a space or a
      tab.  Usually this means a word.
      This matches a string consisting of two equal substrings separated
      by a newline.
      This matches nine characters followed by an `A'.
      This matches the start of a string that contains 16 characters,
      the last of which is an `A'.
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