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Top Forums Shell Programming and Scripting matched characters - regular expression Post 302531334 by jianma on Thursday 16th of June 2011 12:10:38 PM
Old 06-16-2011
matched characters - regular expression

Hi,

I read the book of <<unix shell programming>>. The regular expression ^\(.\)\1 matches the first character on the line and stores it in register 1. Then the expression matches whatever is stored in the register 1, as specified by the \1. The net effect of this regular expression is to match the first two characters on a line if they are both the same character.

I don't fully understand this regular expression, especially "Then the expression matches whatever is stored in the register 1, as specified by the \1." Can someone explain it in detail and more clearly.

Thanks!
 

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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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