Re: [Help-bash] Confused by =~ behavior with regards to quoting

[Jean Delvare]

Hi Greg, hi Pierre,

On Fri, 17 Oct 2014 08:24:29 -0400, Greg Wooledge wrote:
> On Fri, Oct 17, 2014 at 11:37:47AM +0200, Jean Delvare wrote:
> > I'm am confused by the behavior of the =~ operator with regards to
> > quoting, in particular the right hand side:
> 
> Quoted things on the right hand side become string constants, NOT
> regular expressions.  Same with == or = (they become string constants
> instead of glob-style patterns).
> 
> > $ [[ a =~ "a" ]] ; echo $?
> > 0
> 
> You compared the string a and the string a.  They are equal.
> 
> > $ [[ a =~ "^a" ]] ; echo $?
> > 1
> 
> You compared the string a and the string ^a.  They are not equal.

Thanks  (and to Pierre as well) for the quick reply and clear
explanation.

On Fri, 17 Oct 2014 15:24:56 +0300, Pierre Gaston wrote:
> The relevant bit is   "Any part of the pattern may be quoted to force it to
> be matched as a string."
> So, in quotes, the ^ is treated literally and has not special meaning.

It's indeed in the manual page, and I did read that paragraph, but
apparently I stopped reading before that point for some reason. My
bad :-(

On Fri, 17 Oct 2014 08:24:29 -0400, Greg Wooledge wrote:
> The recommendation for people who want to use =~ is to put the regular
> expression into a shell variable, and then use that variable (unquoted)
> on the right hand side.
> 
> re='^gibberish$'
> if [[ $foo =~ $re ]]; then ...

Thanks for the suggestion, I'll remember if I ever need it. That being
said, I can't really think, off the top of my head, of a case where
using the regular expression directly would cause problems. My current
code looks like:

[[ "$1" =~ ^[0-9]+$ ]]

(succeeds if $1 is a positive integer) and it works just fine as far as
I can tell.

-- 
Jean Delvare
SUSE L3 Support