You have a few problems here that we can clear up. First, you need to use warnings. Either include -w on the shebang line of your script:

#!/usr/bin/perl -w
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Or use the "warnings" pragma if your version of Perl supports it:

use warnings;
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These will tell you that "$main::str" was used only once, which can help you see that you have a typo in the following line:

$str = s/.*\-D*?//;
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Further, 'use strict;' will catch errors like that.

You probably meant for $str to be $_. Also, you have an equals '=' sign when you want the regex binding operator: '=~'. The reason you are getting output is because your regular expression is evaluating against the value of $_ and you are throwing away the value of $str. You can either leave off the '$str =' part, or change the code to the following:

$_ = "text1 text2 -f -DFLAG1 -global -DFLAG2 -DFLAG1";
s/.*?(?=-D)//;
print "$_\n";
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The (?=-D) uses a positive lookahead. That checks to see if it matches data, but doesn't include that data in the text that is matched. See perlre for more information on this.

Also, I don't like the dot star in this regular expression. If your data is likely to always be in this format, try the following:

s/[^-]+-[^-]+//;
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However, that regex looks a bit obscure. If this is not a time-sensitive script, you may want to stick with the first one. See Death to Dot Star! for information on problems with the dot star '.*' combination in regexes.

That regular expression, broken out with the /x modifier, reads like this:

s/
  [^-]+  # Negated character class - match one or more
         # of anything that is not a dash: "text1 text2 "
  -      # match the dash: "-"
  [^-]+  # Negated character class - match one or more
         # of anything that is not a dash: "f "
//x;
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No offense, but you seem to have some troubles with English. Since this answer is a bit complicated, feel free to e-mail me at poec@yahoo.com if you have any questions.

Cheers,
Ovid

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