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Re: Regex matching oddity (bug, or 'unexpected feature' or...?)

From: Greg Wooledge
Subject: Re: Regex matching oddity (bug, or 'unexpected feature' or...?)
Date: Thu, 28 May 2009 16:28:20 -0400
User-agent: Mutt/

On Thu, May 28, 2009 at 12:58:10PM -0700, Linda Walsh wrote:
> operator caused a failure in matching.  I.e.:
>       if [[ "$Var"=~"+([:digit:])" ]] ; then ...
> worked.

No, this is incorrect.  You need spaces around the =~ so that it's parsed
as three separate things, rather than one thing.  Your code is being parsed

  if [[ "xxxx" ]]

which is always going to be true, because it's testing whether xxxx is
more than 0 characters long.

> But:
>       if [[ "$Var" =~ "+([:digit:])" ]] ; then ...
> evaluates to 'FALSE', trace(-x) shows the eval as:

If you want the right hand side to be treated as a regular expression,
then you have two problems here:

 1. You're using extglob syntax instead of regex(3) syntax as the man
    page says you should use.

 2. You've quoted the right hand side, which forces it to be treated as
    a literal string instead of a regular expression.

What you probably wanted to write is:

  if [[ $Var =~ $regex ]]; then ...

If you wanted to use the extglob syntax, then you should use = instead of
the =~ operator:

  shopt -s extglob
  if [[ $Var = $glob ]]; then ...

Hope this helps.

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