#!/usr/bin/perl -wl

use strict;
#use re 'eval';

$\= "\n";

my $qred = qr{ MT: (\")? ([^\"]+) (?(1) \"\s | \s) }x;

for( ' MT:yes ' , 'MT:NO" ', 'MT:"yes" ', 'MT:"NO ' ) {
    print;
    print $2 if  /$qred/;
    print '-' x 33;
}

eval q{
    require YAPE::Regex::Explain;
    print YAPE::Regex::Explain->new($qred)->explain;
};
 

__END__

 MT:yes 
yes
---------------------------------
MT:NO" 
---------------------------------
MT:"yes" 
yes
---------------------------------
MT:"NO 
---------------------------------
The regular expression:

(?x-ims: MT: (")? ([^"]+) (?(1) "\s | \s) )

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  MT:                      'MT:'
----------------------------------------------------------------------
  (                        group and capture to \1 (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    "                        '"'
----------------------------------------------------------------------
  )?                       end of \1 (NOTE: because you're using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    [^"]+                    any character except: '"' (1 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  (?(1)                    if back-reference \1 matched, then:
----------------------------------------------------------------------
    "                        '"'
----------------------------------------------------------------------
    \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
   |                        else:
----------------------------------------------------------------------
    \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
  )                        end of conditional on \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------