Question

I have trouble using Perl grep() with a string that may contain chars that are interpreted as regular expressions quantifiers.

I got the following error when the grep pattern is ‘g++’ because the ‘+’ symbols are interpreted as quantifiers. Here is the output of for program that follows:

1..3 ok 1 - grep, pattern not found ok 2 - grep, pattern found  Nested quantifiers in regex; marked by <-- HERE in m/g++ <-- HERE / at escape_regexp_quantifier.pl line 8. 

Is there a modifier I could use to indicate to grep that the quantifiers shall be ignored, or is there a function that would escape the quantifiers ?

#! /usr/bin/perl   sub test_grep($) {     my $filter = shift;     my @output = ('-r-xr-xr-x   3 root     bin       122260 Jan 23  2005 gcc',                   '-r-xr-xr-x   4 root     bin       124844 Jan 23  2005 g++');     return grep (!/$filter/, @output); }  use Test::Simple tests => 2;  ok(test_grep('foo'), 'grep, pattern not found'); ok(test_grep('gcc'), 'grep, pattern found'); ok(test_grep('g++'), 'grep, pattern found'); 

PS: in addition to the answer question above, I welcome any feedback on Perl usage in the above as I’m still learning. Thanks

Answer 1

The standard way is to use the \Q escape indicator before your variable, to tell Perl not to parse the contents as a regular expression:

return grep (!/\Q$filter/, @output); 

Altering that line in your code yields:

 1..3 ok 1 - grep, pattern not found ok 2 - grep, pattern found ok 3 - grep, pattern found