Originally Posted by DGPickett

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\>.*;'

Originally Posted by Scrutinizer

Code:

grep -E 'int .*([^,]+),.*\1[,\;]' infile

Code:

int a,b,b;
int c,d,c;

---------- Post updated at 19:37 ---------- Previous update was at 19:32 ----------

Normal grep:

Code:

grep 'int .*\([^,][^,]*\),.*\1[,\;]' infile