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Top Forums Shell Programming and Scripting matched characters - regular expression Post 302531341 by Skrynesaver on Thursday 16th of June 2011 12:18:59 PM
Old 06-16-2011
If you surround a block within a regular expression with escaped parenthesis (or un-escaped parenthesis when using Perl compatible regex eg. egrep) you are asking the regex parser to remember what was just matched and store it as the next back reference. Thus
Code:
 grep '^\(.\)\1' file

will match any character at the start of a line (the . character)
and store it in the first back reference which can be addressed as \1.
a more obvious example might be where we had a file with records of the form
user homeNode
and we wished to create an internal mailing list
Code:
sed s'/^([^ ]+) ([^ ]+)$/\1@\2/' users_file.txt

This would print out a series of email addresses

Then again re-reading that I'm not sure if it helps, the following may show it more clearly, assuming you've looked at alternation

Another example would be if you wished to match all of a html tag that could contain a ">" character in it (eg. <img alt="Next>" src="/images/next_button.gif"/>)
Code:
<([^>]+|(["'])[^\2]+\2)+>

Here we match anything that is not a ">" character, or anything that is a quoted string which uses either single or double quotes. We capture the quote type in the second set of parenthesis, (the first being the alternation), and then keep matching characters until the end of the quoted string is marked by the quote we previously matched.

Last edited by Skrynesaver; 06-21-2011 at 07:42 AM.. Reason: Second simpler? example
 

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REGEX(3)						     Library Functions Manual							  REGEX(3)

NAME
re_comp, re_exec - regular expression handler SYNOPSIS
char *re_comp(s) char *s; re_exec(s) char *s; DESCRIPTION
Re_comp compiles a string into an internal form suitable for pattern matching. Re_exec checks the argument string against the last string passed to re_comp. Re_comp returns 0 if the string s was compiled successfully; otherwise a string containing an error message is returned. If re_comp is passed 0 or a null string, it returns without changing the currently compiled regular expression. Re_exec returns 1 if the string s matches the last compiled regular expression, 0 if the string s failed to match the last compiled regular expression, and -1 if the compiled regular expression was invalid (indicating an internal error). The strings passed to both re_comp and re_exec may have trailing or embedded newline characters; they are terminated by nulls. The regular expressions recognized are described in the manual entry for ed(1), given the above difference. SEE ALSO
ed(1), ex(1), egrep(1), fgrep(1), grep(1) DIAGNOSTICS
Re_exec returns -1 for an internal error. Re_comp returns one of the following strings if an error occurs: No previous regular expression, Regular expression too long, unmatched (, missing ], too many () pairs, unmatched ). 3rd Berkeley Distribution May 15, 1985 REGEX(3)
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