Bash in-process regular expressions

Bash acquired in-process regular expressions in version 3.0, but I never noticed, probably because most of the machines I'm using are Bash 2.05b. As I'm not necessarily in a position to upgrade any old box I happen to be working on, I tend to stick to the stuff that will work anywhere, which often means piping out to grep for a regular expression test.

However, should you have an environment where you can depend on this feature being present, in-process regexes obviously avoid firing up another process for grep and are much more neat to write or read.

The syntax is Perlish, using "=~". You also have access to sub-matches: $BASH_REMATCH is the string matched, ${BASH_REMATCH[1]} is the first parenthesided match, and so on. So, we can do:

if [[ "$input" =~ 'foo(.*)' ]]
then
        echo $BASH_REMATCH is what I wanted   
        echo but just ${BASH_REMATCH[1]}
fi

and other more interesting tasks.

Set 'nocaseglob' for case insensitivity. The return value for these is 0 for a match, 1 for no match, and (how thoughtful) 2 if the expression is syntactically incorrect, so watch out for the other side of that "if" - a non-zero return might mean you screwed up your pattern.

Comments /Linux/bash-regex.html


Mon Aug 4 05:18:06 2008: Subject:   Taco
www.scheltema.org

When trying this, I could only get it to work without the single quotes:

 

if [[ $input =~ foo(.*) ]] 

then 

        echo $BASH_REMATCH is what I wanted    

        echo but just ${BASH_REMATCH[1]} 

fi 

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