Using a regex saved in an environment variable

J0ax88 has asked for the wisdom of the Perl Monks concerning the following question:

I want to use a regular expression stored in an environment variable. I have this one-liner that works fine without using an environment variable:

perl -00 -ne 'print if /--Begin--/ && /--End--/' file.txt
[download]

file.txt contains:

Hello World

--Begin--
Cat
and
Dog
--End--

other stuff 
This is a test
Good Bye
[download]

The above one-liner prints out what I want:

--Begin--
Cat
and
Dog
--End--
[download]

Now I want to do the same thing except put the regular expression in an environment variable:

export REX="/--Begin--/ && /--End--/"
echo $REX
/--Begin--/ && /--End--/
[download]

So I tried doing this:

perl -00 -ne 'BEGIN {$rex = $ENV{REX}} print if qr($rex)' file.txt
[download]

and also this:

perl -00 -ne 'BEGIN {$rex = qr($ENV{REX})} print if $rex' file.txt
[download]

But it always prints out the whole file. Any help would be appreciated.

Comment on Using a regex saved in an environment variable Select or Download Code

Replies are listed 'Best First'.
Re: Using a regex saved in an environment variable by kcott (Archbishop) on May 03, 2012 at 02:33 UTC
Your problem here is that you're actually using two regexes. Interpolating these separately works fine: `ken@ganymede: ~/tmp $ cat pm_re2_file.txt Hello World --Begin-- Cat and Dog --End-- other stuff This is a test Good Bye ken@ganymede: ~/tmp $ RE1="--Begin--" RE2="--End--" perl -00 -ne 'print if /$ENV{RE1}/ && +/$ENV{RE2}/' pm_re2_file.txt --Begin-- Cat and Dog --End-- ken@ganymede: ~/tmp $` [download] If you have a specific requirement to use just one environment variable, I'd suggest using eval: `ken@ganymede: ~/tmp $ RE1="/--Begin--/ && /--End--/" perl -00 -ne 'print if eval $ENV{RE1} +' pm_re2_file.txt --Begin-- Cat and Dog --End-- ken@ganymede: ~/tmp $` [download] -- Ken	[reply] [d/l] [select]
Re: Using a regex saved in an environment variable by bobf (Monsignor) on May 03, 2012 at 02:13 UTC
I suspect $_ is not what you think it is in the print. Try to get it to work using explicit variables first (one for the file content, one for the regex, and one for the match), then you can watch what happens as you remove them. I don't mean to be cryptic, but I think you're on the right track here...	[reply]
Re: Using a regex saved in an environment variable by JavaFan (Canon) on May 03, 2012 at 08:28 UTC
Now I want to do the same thing except put the regular expression in an environment variable: `export REX="/--Begin--/ && /--End--/" echo $REX /--Begin--/ && /--End--/` [download] But that's not a regexp; that's an expression. You can do the interpolation in the shell, so that Perl sees what you intent it to see: `perl -00 -ne 'print if '$REX file.txt` [download] Here, $REX is exposed to the shell, which sees the above as: `perl -00 -ne 'print if /--Begin--/ && /--End--/' file.txt` [download]	[reply] [d/l] [select]
Re: Using a regex saved in an environment variable by J0ax88 (Initiate) on May 04, 2012 at 01:50 UTC
Thanks Everyone. All the comments were very helpful. This works good: `perl -00 -ne 'print if eval $ENV{REX}' file.txt`	[reply] [d/l]


Think about Loose Coupling
	PerlMonks