Basic	Regular	Expressions
     A regular expression specifies a set of  character	 strings.
     A member of this set of strings is	said to	be matched by the
     regular expression.  Some characters  have	 special  meaning
     when  used	 in  a regular expression; other characters stand
     for themselves.

     The following one-character REs match a single character:

     1.1  An ordinary character	(not one of  those  discussed  in
	  1.2 below) is	a one-character	RE that	matches	itself.

     1.2  A backslash (\) followed by any special character is	a
	  one-character	 RE  that  matches  the	special	character
	  itself.  The special characters are:

	  a.   ., *, [,	and  \	(period,  asterisk,  left  square
	       bracket,	 and  backslash, respectively),	which are
	       always special, except  when  they  appear  within
	       square brackets ([]; see	1.4 below).
               Also ? and + are special in Perl.

	  b.   ^ (caret	or circumflex),	which is special  at  the
	       beginning of an entire RE (see 4.1 and 4.3 below),
	       or when it immediately follows the left of a  pair
	       of square brackets ([]) (see 1.4	below).

	  c.   $ (dollar sign),	which is special at the	end of an
	       entire RE (see 4.2 below).

	  d.   The character used to bound (that is, delimit)  an
	       entire RE, which	is special for that RE (for exam-
	       ple, see	how slash (/) is used in the  g	 command,
	       below.)

     1.3  A period (.) is a one-character  RE  that  matches  any
	  character except new-line.

     1.4  A non-empty string of	 characters  enclosed  in  square
	  brackets  ([])  is  a	one-character RE that matches any
	  one character	in that	string.	 If, however,  the  first
	  character  of	 the string is a circumflex (^), the one-
	  character RE matches any character except new-line  and
	  the remaining	characters in the string.  The ^ has this
	  special meaning only if it occurs first in the  string.
	  The  minus  (-) may be used to indicate a range of con-
	  secutive characters; for example, [0-9]  is  equivalent
	  to  [0123456789].   The - loses this special meaning if
	  it occurs first (after an initial ^, if any) or last in
	  the string.  The right square	bracket	(]) does not ter-
	  minate such a	string when it	is  the	 first	character
	  within  it  (after  an initial ^, if any); for example,
	  []a-f] matches either	a right	square bracket (]) or one
	  of  the  ASCII letters a through f inclusive.	 The four
	  characters listed in 1.2.a above stand  for  themselves
	  within such a	string of characters.

     The following rules may be	used to	construct REs  from  one-
     character REs:

     2.1  A one-character RE is	a RE that  matches  whatever  the
	  one-character	RE matches.

     2.2  A one-character RE followed by an asterisk (*) is a  RE
	  that matches 0 or more occurrences of	the one-character
	  RE.  If there	 is  any  choice,  the	longest	 leftmost
	  string that permits a	match is chosen.

     2.2' A one-character RE followed by a plus sign (+) matches
          one or more occurrences of the one-character RE.

     2.3  A  one-character  RE	followed  by  \{m\},  \{m,\},  or
	  \{m,n\}  is a	RE that	matches	a range	of occurrences of
	  the one-character RE.	 The values of m and  n	 must  be
	  non-negative	integers  less	than  256;  \{m\} matches
	  exactly  m  occurrences;  \{m,\}  matches  at	 least	m
	  occurrences;	\{m,n\}	matches	any number of occurrences
	  between m and	n inclusive.  Whenever a  choice  exists,
	  the RE matches as many occurrences as	possible.

     2.3' A one-character RE followed by question mark (?) matches
          zero or one occurrence of the RE.  In other words, this
          indicates the RE is optional.

     2.4  The concatenation of REs is a	RE that	matches	the  con-
	  catenation  of the strings matched by	each component of
	  the RE.

     2.5  A RE enclosed	between	the character sequences	\( and \)
	  is a RE that matches whatever	the unadorned RE matches.

     2.6  The expression \n matches the	same string of characters
	  as was matched by an expression enclosed between \( and
	  \) earlier in	the same RE.  Here  n  is  a  digit;  the
	  sub-expression  specified is that beginning with the n-
	  th occurrence	of \( counting from the	left.  For  exam-
	  ple,	the expression ^\(.*\)\1$ matches a line consist-
	  ing of two repeated appearances of the same string.

     A RE may be constrained to	match words.

     3.1  \< constrains	a RE to	match the beginning of	a  string
	  or  to  follow  a character that is not a digit, under-
	  score, or letter.  The first character matching the  RE
	  must be a digit, underscore, or letter.

     3.2  \> constrains	a RE to	match the end of a string  or  to
	  precede a character that is not a digit, underscore, or
	  letter.

     An	entire RE may be constrained to	 match	only  an  initial
     segment or	final segment of a line	(or both).

     4.1  A circumflex (^) at the beginning of an entire RE  con-
	  strains that RE to match an initial segment of a line.

     4.2  A dollar sign	($) at the end of an entire RE constrains
	  that RE to match a final segment of a	line.

     4.3  The construction ^entire RE$ constrains the  entire  RE
	  to match the entire line.