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Full Discussion: grep backreferencing question
Top Forums Shell Programming and Scripting grep backreferencing question Post 302464185 by DGPickett on Tuesday 19th of October 2010 01:28:52 PM
Old 10-19-2010
Your grep is "too anchored" and your regex visualization is too wild. There is no back referencing in regex, just iteratively forward testing: '.*' means try remainder of pattern at every following byte.

A line containing the word int and later a semicolon should not have any variable-legal word repeated between them. Every variable name in C must start with a letter, the rest of the name can consist of letters, numbers and underscore characters. Commas are not variable-legal words, so you can ignore them -- classic excess information problem.

Deal with white spaces using \<\> or similar word boundary, so you avoid substrings but do not get tangled in the whole comma, space, tab thing. Some grep do not honor '\<\>' so you may need sed or '\b'.

Regex Tutorial - \b Word Boundaries

If you get desperate, add spaces by commas and semicolon so you can look for space or tab [ \t]. If you need to restore the original, sed has a hold space h/g command pair.

Code:
Narrative: grep for a line with the free standing word 'int', and
 later on that line for every C variable name as a free standing word somewhere,
  see if we have that same C variable name as a free standing word later anywhere,
 and yet later on that line a semicolon.

grep '\<int\>.*\<\([a-zA-Z][a-zA-Z0-9_]*\)\>.*\<\1\>.*;'

 

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LEARN ABOUT DEBIAN

xzgrep

XZGREP(1)							     XZ Utils								 XZGREP(1)

NAME

       xzgrep - search compressed files for a regular expression

SYNOPSIS

       xzgrep [grep_options] [-e] pattern file...
       xzegrep ...
       xzfgrep ...
       lzgrep ...
       lzegrep ...
       lzfgrep ...

DESCRIPTION

       xzgrep  invokes	grep(1)  on  files which may be either uncompressed or compressed with xz(1), lzma(1), gzip(1), bzip2(1), or lzop(1).  All
       options specified are passed directly to grep(1).

       If no file is specified, then standard input is decompressed if necessary and fed to grep(1).  When reading from standard  input,  gzip(1),
       bzip2(1), and lzop(1) compressed files are not supported.

       If  xzgrep  is  invoked as xzegrep or xzfgrep then egrep(1) or fgrep(1) is used instead of grep(1).  The same applies to names lzgrep, lze-
       grep, and lzfgrep, which are provided for backward compatibility with LZMA Utils.

ENVIRONMENT

       GREP   If the GREP environment variable is set, xzgrep uses it instead of grep(1), egrep(1), or fgrep(1).

SEE ALSO

       grep(1), xz(1), gzip(1), bzip2(1), lzop(1), zgrep(1)

Tukaani 							    2011-03-19								 XZGREP(1)
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