Passing a regex substitution as a variable in Perl

Question

I need to pass a regex substitution as a variable:

sub proc {     my $pattern = shift;     my $txt = "foo baz";      $txt =~ $pattern; }  my $pattern = 's/foo/bar/'; proc($pattern); 

This, of course, doesn't work. I tried eval'ing the substitution:

eval("$txt =~ $pattern;"); 

but that didn't work either. What horribly obvious thing am I missing here?

Answer 1

I need to pass a regex substitution as a variable

Do you? Why not pass a code reference? Example:

sub modify {   my($text, $code) = @_;   $code->($text);   return $text; }  my $new_text = modify('foo baz', sub { $_[0] =~ s/foo/bar/ }); 

In general, when you want to pass "something that does something" to a subroutine ("a regex substitution" in the case of your question) the answer is to pass a reference to a piece of code. Higher Order Perl is a good book on the topic.

Answer 2

sub proc {     my($match, $subst) = @_;     my $txt = "foo baz";     $txt =~ s/$match/$subst/;     print "$txt\n"; }  my $matcher = qr/foo/; my $sub_str = "bar";  proc($matcher, $sub_str); 

This rather directly answers your question. You can do more - but when I used a qr// term instead of the $sub_str as a simple literal, then the expanded regex was substituted.

I recently needed to create a parser (test parser) for statements with some peculiar (dialect of) SQL types, recognizing lines such as this, splitting it into three type names:

input: datetime year to second,decimal(16,6), integer 

The script I used to demo this used quoted regexes.

#!/bin/perl -w use strict; while (<>) {     chomp;     print "Read: <$_>\n";     my($r1) = qr%^input\s*:\s*%i;     if ($_ =~ $r1)     {         print "Found input:\n";         s%$r1%%;         print "Residue: <$_>\n";         my($r3) = qr%(?:year|month|day|hour|minute|second|fraction(?:\([1-5]\))?)%;         my($r2) = qr%                         (?:\s*,?\s*)?   # Commas and spaces                         (                             (?:money|numeric|decimal)(?:\(\d+(?:,\d+)?\))?   |                             int(?:eger)?  |                             smallint      |                             datetime\s+$r3\s+to\s+$r3                         )                     %ix;         while ($_ =~ m/$r2/)         {             print "Got type: <$1>\n";             s/$r2//;         }         print "Residue 2: <$_>\n";     }     else     {         print "No match:\n";     }     print "Next?\n"; } 

We can argue about the use of names like $r1, etc. But it did the job...it was not, and is not, production code.