Re: Regular expression expanding and matching

[Peter Stephenson <p.w.stephenson@ntlworld.com>]

On Sun, 25 Nov 2012 16:45:49 +0100
Mark van Dijk <lists+zsh@internecto.net> wrote:
> I was trying to match a string with a regular expression as follows:
> 
> ---
> #!/usr/local/bin/zsh
> zmodload zsh/pcre
> somestring="121125"
> todaysday="25"
> yesterday="24"
> 
> set -xv
> if [[ $somestring -pcre-match \d{4}${todaysday} ]]; then
>     echo "somestring matches today"
> elif [[ $somestring -pcre-match \d{4}${yesterday} ]]; then
>     echo "somestring matches yesterday"
> fi
> set +xv
> 
> Apparently there is no expansion of ${todaysday} and ${yesterday}. This
> is not really surprising because in regular expressions many characters
> have a different meaning.

I think the problem is different from what you think it is.  Try (note
doubled backslashes):

if [[ $somestring -pcre-match \\d{4}${todaysday} ]]; then
    echo "somestring matches today"
elif [[ $somestring -pcre-match \\d{4}${yesterday} ]]; then
    echo "somestring matches yesterday"
fi

It's not that the $... isn't expanded, it's that the \d is also handled
like a normal command line argument.  Vin's change works because in
double quotes \d remains \d because it doesn't have a special meaning.

To put it more broadly, arguments in [[ ... ]] get expanded in all the ways that make sense for generating a single word.  So file name generation (globbing)
doesn't happen, and array substitution produces a single word (as if double
quotes surrounded the array), but otherwise it works like a normal command line argument.  So the normal quoting rule for backslashes apply.

-- 
Peter Stephenson <p.w.stephenson@ntlworld.com>
Web page now at http://homepage.ntlworld.com/p.w.stephenson/