bash: using =~ with regex containing #

Question

Alright folks. I am writing a bash script and would like compare a string against a regular expression containing a # using the =~ operator.

Here is what I have so far:

if [[ ${line} =~ \s*\# ]]; then
     #do things
fi

As you can see, I am attempting to escape the # with a \, which is supposed to be possible according to this article. This is really confounding to me, however. My syntax highlighter is still highlighting the text following the # as if it were a comment.

Is my syntax highlighter incorrect? Will the escape of the # interfere with the parsing of the regex? Is there some way I can use quotes to avoid this issue?

Expecting your editor's syntax highlighter to get this right is, frankly, asking too much of it. @anubhava's answer is a good one, but it's a fix for your editor's highlighter (and for consistent behavior with really old versions of bash that are never seen in practice anymore), not something with impact on shell behavior on modern implementations. — Charles Duffy
– Charles Duffy, Commented Jul 2, 2015 at 20:44
Incidentally, # is never a shell comment character when it exists partway through a word, so your editor is really, really off here. — Charles Duffy
– Charles Duffy, Commented Jul 2, 2015 at 20:46
Hmm. If I weren't busy right now, I'd be tempted to go submit a patch... — Charles Duffy
– Charles Duffy, Commented Jul 2, 2015 at 20:50

anubhava · Accepted Answer · 2015-07-02 20:43:29Z

5

You can do:

re='\s*#'

if [[ $line =~ $re ]]; then
     #do things
fi

answered Jul 2, 2015 at 20:43

anubhava

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4 Comments

scottysseus Over a year ago

great, thanks. I wasn't sure if quotes would work properly with the =~ operator... I still don't fully understand their use in control flow statements

anubhava Over a year ago

Quoted text doesn't work on RHS of =~ operator but variables can be used as I showed above.

chepner Over a year ago

A quoted string simply escapes every character ("foo" and \f\o\o produce identical strings, for instance). It's the result after quote removal that is assigned to the variable, so there's no way to tell if the assignment was re="foo" or re=\f\o\o or re="of"o or any other possibility: the end result is a variable containing a string of 3 characters.

chepner Over a year ago

(Also, you can quote some text on the RHS, just not any characters that are intended as metacharacters. [[ foo =~ "fo*" ]] won't succeed, but [[ foo =~ "fo"* ]] would match as expected.)

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