Regex match for whole string (not substring) in bash

Question

I want this to only output "match!" if only the single character "a" or "b" is passed as an argument. Not aaaaa, not bcfqwefqef, not qwerty.

#!/bin/bash
P="a|b"
if [[ "$1" =~ $P  ]]; then
    echo "match!"
else
    echo "no!"
fi

Yes i've gone through some SO posts to get this far already. Putting $P in quotes doesn't work either.

anubhava · Accepted Answer · 2017-05-26 20:19:55Z

6

You need to anchor your regex:

#!/bin/bash

re="^(a|b)$"
if [[ "$1" =~ $re ]]; then
    echo "match!"
else
    echo "no!"
fi

btw this doesn't require regex. You can just use equality using glob pattern as:

if [[ "$1" == [ab] ]]; then
    echo "match!"
else
    echo "no!"
fi

answered May 26, 2017 at 20:19

anubhava

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4 Comments

atxdba Over a year ago

The ^(...)$ wrapping is what i needed, the real world use case was strings longer than a and b

chepner Over a year ago

You might still be able to use pattern matching: "$1 == (long string a|long string b|long string c), for instance. Pattern matching is typically simpler if the choices are quite specific.

Benjamin W. Over a year ago

@chepner Needs @(pattern1|pattern2), no?

chepner Over a year ago

Yeah, I was thinking of case syntax.

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