neversaint: 
what's wrong with my script above such that it prints no underscore instead of with underscore

There have been already correcting hints in all directions by moritz and CountZero. From analyzing your code, jasonk pointed out your misconception on /x and whitespace, which means your code would work as intended if you change the regex modifier to:

 ...
 if ( $str =~ / / ) {  
    print "no underscore\n";  
 }
 else {
    print "with underscore\n";
 }
 ...

[download]

The /x modifier would lead the regex to ignore the space (as has been said) and the /m and /s aren't needed here (they don't do anything)

In another response, grinder scrutinized your problem solution and offered a more efficient solution based on the index() function without any regular expressions.

In addition to these hints, davido tackles the problem by an important feature of the tr// (transliteration) operator - to count occurrences of characters in very efficient way. This would reduce your problem to the following expression:

  ...
  my $str = $ARGV[0] || '|78187980|ref|NM_0';          # original stri
+ng

  my $cnt = $str =~ tr/_//;                            # count the num
+ber of underscores

  print 'with ' . ($cnt || 'no') . " underscore(s)\n"; # print result 
+depending on count
  ...

[download]

after which you may decide on the 'count' of the character in question.

Regards

mwa

In reply to Re: Regex for Differentiating Underscore and Whitespace by mwah
in thread Regex for Differentiating Underscore and Whitespace by neversaint

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