Re^3: Using Look-ahead and Look-behind

[AnomalousMonk]

Here's a solution that exactly matches the phrases specified in AnonyMonk's Re: Using Look-ahead and Look-behind post (which the code of Re^2: Using Look-ahead and Look-behind does not quite do), and also shows how to use the newfangled backtracking control verbs of 5.10 to emulate variable-width negative look-behind. Variable-width positive look-behind is emulated by 5.10's  \K assertion.

Explanation:

• Any 'equity' that is preceded by
  • either a character that is not a comma or whitespace, or
  • by the 'private' phrase
  FAILS and is skipped over (this test has first precedence);
• Otherwise, any 'equity' that is not followed by a comma that is then followed by any non-whitespace SUCCEEDS.

>perl -wMstrict -le
"use Test::More 'no_plan';
 ;;
 for my $ar_vector (
   [ YES => 'equity, private equity', ],
   [ YES => 'equity',                 ],
   [ no  => 'private equity',         ],
   [ YES => 'private equity,equity',  ],
   [ YES => 'private equity, equity', ],
   [ no  => 'equity,private equity',  ],
   [ no  => 'private equity',         ],
   [ no  => 'mutual funds',           ],
   [ no  => 'cds'                     ],
   ) {
   my ($expected, $string) = @$ar_vector;
   is match($string), $expected, qq{'$string'};
   }
 ;;
 sub match {
   my ($string) = @_;
   ;;
   my $char_not_comma_or_space = qr{ [^,\s]      }xms;
   my $private                 = qr{ private \s+ }xms;
   return 'YES' if $string =~
     m{ (?: $char_not_comma_or_space | $private) equity (*SKIP)(*FAIL)
        |
        equity (?! , \S)
      }xms;
   return 'no',
   }
"
ok 1 - 'equity, private equity'
ok 2 - 'equity'
ok 3 - 'private equity'
ok 4 - 'private equity,equity'
ok 5 - 'private equity, equity'
ok 6 - 'equity,private equity'
ok 7 - 'private equity'
ok 8 - 'mutual funds'
ok 9 - 'cds'
1..9

[JohnN]

I have a dumb question.

This code works well (THANKS Roy!) when looking for DNA string matches within a genome sequence but not when the * is changed to {50,100}

e.g.


/CCGG          # Match starting at DNA sequence CCGG
  (          
    (?:        
      (?!CCGG) #   make sure we're not finding duplicates mid-stream
      .             #   accept any character
    )*?            # any number of times BUT not greedily <====
  )          
  AATT         # and ending at AATT
/x;

versus

/CCGG   
  (          
    (?:        
      (?!CCGG)  
      .        
    ){50,100}?        # <====
   )         
  AATT  # and ending at AATT
/x;

This latter one does not have dupes of CCGG but does have dupes of AATT. The previous snippet has no dupes of either CCGG or AATT.

A follow-up: The following code snippet fixes my problem, and I have NO idea why! I tried it out of desperation

/CCGG   
  (          
    (?:        
      (?!AATT|CCGG) #     <=============
      .       # 
    ){50,100}?        # Here the "?" is not required but I'm anal
  )         # 
  AATT  # 
/x;

[choroba]

When * is changed to ^, it does not work either. Why are you changing it at all?

But jokes aside: The *? matches after seeing the first occurence of AATT, so there are no dupes. The {50,100} must match at least 50 times, so if there is AATT after say 25^th character, it cannot stop there and must match a larger string.

Use YAPE::Regex::Explain to see what your regular expresions mean.

Moreover, you are replying to a node that is not related to your question.

[Anonymous Monk]

Great, please take it to Seekers Of Perl Wisdom, see Re^2: Using Look-ahead and Look-behind

You forgot to include sample input, no matter, here are clues, run these and compare

perl -Mre=debug -le " $_ = q/foobarfoodrinkAATT/; /foo((?:(?!bar).){1,
+5}?)AATT/; "

perl -Mre=debug -le " $_ = q/foobarfoodrinkAATTAATT/; /foo((?:(?!bar).
+){6,10}?)AATT/; "

50,100 means match at minimum 50 but no more than 100

.* means match at least zero times

in my short example, first AATT appears at 6, so it is included in the match