Question

In bash, I am trying to match valid attributes that are present in an array. Attributes may be ‘disabled’ by preceding them with a bang (exclamation mark, !), in which case they must not be matched. I have this:

[[ ${TESTS[@]} =~ [^\!]match ]]

which will return true if the word ‘match’ is in TESTS and not preceded by a !.

It works, except when the word match is in the first position in the array. The problem is the regexp is saying ‘match preceded by something that isn’t a !’. When it’s the first item it is preceded by nothing and therefore does not match.

How do I modify the above to say ‘match not preceded by !’ ?

From reading answers to other questions I have tried (?<!!)match but this does not work.

Answer 1

Use this re:

([^\!]|^)match

Example of usage:

$ [[ match =~ (^|[^\!])match ]] && echo matches || echo "doesn't match"
matches
$ [[ xmatch =~ (^|[^\!])match ]] && echo matches || echo "doesn't match"
matches
$ [[ '!match' =~ (^|[^\!])match ]] && echo match || echo "doesn't match"
doesn't match

In general, it would be also correct to use assertions here, but bash uses POSIX regular expressions and they know nothing about assertions. But with grep (GNU grep), or perl, or anything that supports PCRE you can do it:

$ echo match | grep -qP '(?<!!)match' && echo matches || echo "doesn't match"
matches
$ echo xmatch | grep -qP '(?<!!)match' && echo matches || echo "doesn't match"
matches
$ echo '!match' | grep -qP '(?<!!)match' && echo matches || echo "doesn't match"
doesn't match