When working with regular expressions in a shell script the norm is to use grep or sed or some other external command/program. Since version 3 of bash (released in 2004) there is another option: bash's built-in regular expression comparison operator "=~".

Bash's regular expression comparison operator takes a string on the left and an extended regular expression on the right. It returns 0 (success) if the regular expression matches the string, otherwise it returns 1 (failure).

In addition to doing simple matching, bash regular expressions support sub-patterns surrounded by parenthesis for capturing parts of the match. The matches are assigned to an array variable BASH_REMATCH. The entire match is assigned to BASH_REMATCH[0], the first sub-pattern is assigned to BASH_REMATCH[1], etc..

The following example script takes a regular expression as its first argument and one or more strings to match against. It then cycles through the strings and outputs the results of the match process:

#!/bin.bash

if [[ $# -lt 2 ]]; then
    echo "Usage: $0 PATTERN STRINGS..."
    exit 1
fi
regex=$1
shift
echo "regex: $regex"
echo

while [[ $1 ]]
do
    if [[ $1 =~ $regex ]]; then
        echo "$1 matches"
        i=1
        n=${#BASH_REMATCH[*]}
        while [[ $i -lt $n ]]
        do
            echo "  capture[$i]: ${BASH_REMATCH[$i]}"
            let i++
        done
    else
        echo "$1 does not match"
    fi
    shift
done

Assuming the script is saved in "bashre.sh", the following sample shows its output:

  # sh bashre.sh 'aa(b{2,3}[xyz])cc' aabbxcc aabbcc
  regex: aa(b{2,3}[xyz])cc

  aabbxcc matches
    capture[1]: bbx
  aabbcc does not match

Mitch Frazier is an Associate Editor at Linux Journal.