/x modifier with \Q and \E

wardy3 has asked for the wisdom of the Perl Monks concerning the following question:

Hi all. I think I'm missing something simple but I just can't see it.

I am trying to split a string on markers that I don't want to hard-code. My patterns aren't behaving - here's a simple stub of what is throwing me.

use strict;

my $patt1 = '!!';
my $patt2 = '++';

my $string = 'hello!!there++everyone!!in the web';

if ( $string =~ m{$patt1} ) {
    print "Matches patt1\n";
}
if ( $string =~ m{\Q $patt2 \E}x ) {
    print "Matches patt2 with /x\n";
}
if ( $string =~ m{\Q$patt2\E} ) {
    print "Matches patt2\n";
}
[download]

I would have thought this would have matched all 3 times but the /x modifier doesn't match it. Does the \Q\E make the spaces significant?

Can someone see why?

Thanks!

Comment on /x modifier with \Q and \E Download Code

Replies are listed 'Best First'.
Re: /x modifier with \Q and \E by NetWallah (Canon) on Apr 01, 2008 at 05:37 UTC
From perlre: Also note that anything inside a \Q...\E stays unaffected by /x. In other words, whitespace IS significant inside quotemeta, regardless of /x. "As you get older three things happen. The first is your memory goes, and I can't remember the other two... " - Sir Norman Wisdom	[reply]
Re^2: /x modifier with \Q and \E by wardy3 (Scribe) on Apr 01, 2008 at 05:47 UTC
aaaaaahhh! Thanks My version of perldoc doesn't have that sentence! I just checked online and the sentence is there. Must have been updated since my Perl. Thanks again.	[reply]
Re^2: /x modifier with \Q and \E by parv (Parson) on Apr 01, 2008 at 05:58 UTC
(I missed OP's reply which stated the same as below.) I could not find that line in my copies of perlre or perlop (perl 5.8.7) ... { perldoc perlre ; perldoc perlop ; } \| grep -C 2 unaffected just as in ordinary Perl code. This also means that if you wan +t real whitespace or "#" characters in the pattern (outside a characte +r class, where they are unaffected by "/x"), that you'll either have to +escape them or encode them using octal or hex escapes. Taken together +, these features go a long way towards making Perl's regular expression +s more -- cards, pipes, and redirections will be honored. The co +llected standard output of the command is returned; standard er +ror is unaffected. In scalar context, it comes back as a sing +le (potentially multi-line) string, or undef if the comman +d failed. In list context, returns a list of lines (howe +ver [download]	[reply] [d/l]
Re: /x modifier with \Q and \E by parv (Parson) on Apr 01, 2008 at 05:27 UTC
`/x` apparently has no effect. `\Q` is what affects the space (for `\Q` implies to quote any characters following it, till the end or upto `\E` if present). # Need perl to be built with DEBUGGING perl -Dr -e '$p = qr{\Q ++ \E} ; $q = qr{\Q ++ \E}x ; $r = qr{\Q++\E}' Compiling REx `\ \+\+\ ' size 3 Got 28 bytes for offset annotations. first at 1 rarest char + at 1 1: EXACT < ++ >(3) 3: END(0) anchored " ++ " at 0 (checking anchored isall) minlen 4 Offsets: [3] 1[8] 0[0] 9[0] Compiling REx `\ \+\+\ ' size 3 Got 28 bytes for offset annotations. first at 1 rarest char + at 1 1: EXACT < ++ >(3) 3: END(0) anchored " ++ " at 0 (checking anchored isall) minlen 4 Offsets: [3] 1[8] 0[0] 9[0] Compiling REx `\+\+' size 3 Got 28 bytes for offset annotations. first at 1 rarest char + at 0 1: EXACT <++>(3) 3: END(0) anchored "++" at 0 (checking anchored isall) minlen 2 Offsets: [3] 1[4] 0[0] 5[0] Omitting $` $& $' support. EXECUTING... Freeing REx: `"\\ \\+\\+\\ "' Freeing REx: `"\\ \\+\\+\\ "' Freeing REx: `"\\+\\+"' [download] I consulted `perlre` to see if there was anything about a space with \Q & /x. Finding nothing there, I moved to the pointer provided to Gory details of parsing quoted constructs in `perlop`. That section also did not list anything about the topic. The reading of that section, however, was interesting. All was not in vain.	[reply] [d/l] [select]
Re^2: /x modifier with \Q and \E by wardy3 (Scribe) on Apr 01, 2008 at 05:40 UTC
Thanks parv - I thought I'd try the regex debugger too just before I posted the message. I saw that it was trying to anchor ' ++ ', which prompted my question at the end of the original post. It surprised me and I thought I'd lost the plot somewhere along the line. I wanted to make it possible for my script to handle special regex characters (like the +) so at this point, I think I'll just write cluttered regex's unless someone can point out how to do it :-) Thanks	[reply]
Re^3: /x modifier with \Q and \E by parv (Parson) on Apr 01, 2008 at 05:54 UTC
I suppose you are aware of quotemeta function, which you could use for the same effect of `\Qpattern\E` something like (one of many variations) ... `my @patterns = map { quotemeta $_ } @string` [download] In any case, as the Gory details says, be aware of what happens with `[$@]` in `\Q` (or `quotemeta`) including other meta characters.	[reply] [d/l] [select]
Re^4: /x modifier with \Q and \E by wardy3 (Scribe) on Apr 01, 2008 at 08:32 UTC