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Full Discussion: Strange RegExp Behaviour
Top Forums Shell Programming and Scripting Strange RegExp Behaviour Post 302502192 by itskov on Monday 7th of March 2011 07:05:52 AM
Old 03-07-2011
Bug Strange RegExp Behaviour

Hello,

I was trying to identify lines who has a word of the following pattern "xyyx" (where x, and ys are different characters).

I was trying the following grep -
Code:
egrep '(\S)([^\1])\2\1'

This pattern do catches the wanted pattern, but it also catches "GGGG" or "CCCC" patterns. I was trying to check this on different regex infrastructure and it worked fine (catched only the wanted pattern).

Does anyone has an idea how to change the regex so it catches only the wanted pattern?

To summarize the problem - you can check -
Code:
echo 'GGGGGG' | egrep '(\S)([^\1]\2\1'

Thanks a lot in advance!
Eyal.

Last edited by Franklin52; 03-07-2011 at 08:16 AM.. Reason: Please use code tags, thank you
 

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GREP(1) 						      General Commands Manual							   GREP(1)

NAME
grep, g - search a file for a pattern SYNOPSIS
grep [ option ... ] pattern [ file ... ] g [ option ... ] pattern [ file ... ] DESCRIPTION
Grep searches the input files (standard input default) for lines that match the pattern, a regular expression as defined in regexp(7) with the addition of a newline character as an alternative (substitute for |) with lowest precedence. Normally, each line matching the pattern is `selected', and each selected line is copied to the standard output. The options are -c Print only a count of matching lines. -h Do not print file name tags (headers) with output lines. -e The following argument is taken as a pattern. This option makes it easy to specify patterns that might confuse argument parsing, such as -n. -i Ignore alphabetic case distinctions. The implementation folds into lower case all letters in the pattern and input before interpre- tation. Matched lines are printed in their original form. -l (ell) Print the names of files with selected lines; don't print the lines. -L Print the names of files with no selected lines; the converse of -l. -n Mark each printed line with its line number counted in its file. -s Produce no output, but return status. -v Reverse: print lines that do not match the pattern. -f The pattern argument is the name of a file containing regular expressions one per line. -b Don't buffer the output: write each output line as soon as it is discovered. Output lines are tagged by file name when there is more than one input file. (To force this tagging, include /dev/null as a file name argument.) Care should be taken when using the shell metacharacters $*[^|()= and newline in pattern; it is safest to enclose the entire expression in single quotes '...'. An expression starting with '*' will treat the rest of the expression as literal characters. G invokes grep with -n and forces tagging of output lines by file name. If no files are listed, it searches all files matching *.C *.b *.c *.h *.m *.cc *.java *.cgi *.pl *.py *.tex *.ms SOURCE
/src/cmd/grep /bin/g SEE ALSO
ed(1), awk(1), sed(1), sam(1), regexp(7) DIAGNOSTICS
Exit status is null if any lines are selected, or non-null when no lines are selected or an error occurs. GREP(1)
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