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Top Forums UNIX for Dummies Questions & Answers Need help with Regex for bash Post 302750037 by Don Cragun on Sunday 30th of December 2012 03:54:26 PM
Old 12-30-2012
No, expr and egrep do not use the same types of regular expression. Although some man pages aren't very clear about what form of regular expressions are used by expr match string pattern and expr string : pattern, the standards are clear:
Code:
       The  ':'  matching operator shall compare the string resulting from the
       evaluation of expr1 with the regular expression pattern resulting  from
       the  evaluation	of  expr2.  Regular  expression  syntax  shall be that
       defined in the Base Definitions volume of IEEE Std 1003.1-2001, Section
       9.3,  Basic  Regular Expressions, except that all patterns are anchored
       to the beginning of the string (that is, only sequences starting at the
       first character of a string are matched by the regular expression) and,
       therefore, it is unspecified whether '^' is a special character in that
       context.  Usually,  the	matching operator shall return a string repre-
       senting the number of characters matched ( '0'  on  failure).  Alterna-
       tively,	if the pattern contains at least one regular expression subex-
       pression "[\(...\)]" ,  the  string  corresponding  to  "\1"  shall  be
       returned.

Note that the match string pattern form is not in the standards; it is an extension supported on some (but not all) systems.

The egrep (which is obsolete) and the equivalent grep -E commands use extended regular expressions. The description of grep's regular expressions in POSIX is:
Code:
Regular expression matching shall be based on text lines. Since a <newline> separates or
terminates patterns (see the −e and −f options below), regular expressions cannot contain a
<newline>. Similarly, since patterns are matched against individual lines (excluding the
terminating <newline> characters) of the input, there is no way for a pattern to match a
<newline> found in the input.

and
Code:
       -E     Match using extended regular  expressions.  Treat  each  pattern
	      specified as an ERE, as described in the Base Definitions volume
	      of IEEE Std 1003.1-2001, Section 9.4, Extended  Regular  Expres-
	      sions.   If any entire ERE pattern matches some part of an input
	      line excluding the terminating  <newline>,  the  line  shall  be
	      matched.	A null ERE shall match every line.

You don't need to backslash-escape characters in a bracket expression in an RE. If you do, the backslash character itself becomes a member of the set of characters to be matched by a matching bracket expression or to be excluded from the list of characters to be matched by a non-matching bracket expression.
This User Gave Thanks to Don Cragun For This Post:
 

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