Re^6: regex return true instead of false (precedence)

[AnomalousMonk]

if( $args !~ / (*FAIL)
   | \-rs(\s+)(\S+) 
#  | \-? (\s+)(\S+)
   | \-P (\s+)(\S+)
   | \-p (\s+)(\S+)
   /x ){ ... }

I don't understand the purpose of the  (*FAIL) operator (see Special Backtracking Control Verbs in perlre in Perl versions 5.10+ (update: see also Backtracking control verbs in perlretut - but that's not actually as extensive as the discussion in perlre)) in the quoted regex. At any position in the  | alternation,  (*FAIL) will simply force the RE to try the next alternation; if it's at the last position, all the preceding pattern matches would have failed and the alternation would fail anyway without (*FAIL).

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 for my $s (qw(XXX ppp)) {
   print qq{for '$s' string};
   if ($s !~ m{ (*F) | p | q }xms) { print '  with (*F): no match, !~ 
+true' }
   if ($s !~ m{   Z  | p | q }xms) { print '  with Z:    no match, !~ 
+true' }
   if ($s !~ m{        p | q }xms) { print '  no (*F):   no match, !~ 
+true' }
   print '  -------';
   }
"
for 'XXX' string
  with (*F): no match, !~ true
  with Z:    no match, !~ true
  no (*F):   no match, !~ true
  -------
for 'ppp' string
  -------

Note that for all these variations, for string 'XXX' there is never a match (!~ is always true); string 'ppp' always matches (!~ always false).

... not equivalently, because there are 8 capturing groups instead of 2 ...

If you're using  (*FAIL) you must be using Perl version 5.10+, so you also have the  (?|pattern) "branch reset" operator (see Extended Patterns in perlre). This allows you to go back to having two capture groups again!

Update: Changed example code slightly to better reflect discussion.

[rsFalse]

"I don't understand the purpose of the (*FAIL) operator" -- AnomalousMonk.

It has the same purpose as zero in separated regex example. It allows '||' or '|' be written before first conditional. And then all conditionals have the same line-format. For me it is more readable :)

It is similar to:

if( 0 ){ ; }
elsif( condition_1 ){ ... }
elsif( condition_2 ){ ... }
elsif( condition_3 ){ ... }

...which is sometimes a bit more readable. Also then I can easily change the sequence of conditions by only swapping lines (including the line with the first condition).