Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

How to write an efficient Perl script to return the balance change of the string?

Balance Change:


'abc abc abc \textit{abc abc} abc \textit{abc abc \textbf{abc \textbf{ +abc abc} abc} abc} abc.'<br>


'abc abc abc <i>abc abc</i> abc <i>abc abc <b>abc <b>abc abc</b> abc</b> abc</i> abc.'


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Re: Balance Change
by Mago (Parson) on Feb 08, 2006 at 16:47 UTC

    perl -e '$_=shift;X while s/\\text(i|b)[tf]\{([^{}]*)\}/<$1>$2<\/$1>/g +;print;' 'abc abc abc \textit{abc abc} abc \textit{abc abc \textbf{ab +c \textbf{abc abc} abc} abc} abc.'


      Ok, I understand all of this except for the 'x while.' I understand that x is a repetition operator but I've always seen it written like: '"a" x 80.' However here, there is nothing on the left hand side. Could you explain what it does? I'm guessing it repeats the substitution so that it can find the nested cases but I'd be interested in knowing how it works.

        It's not the (lower-case) x operator, it's an (upper-case) unquoted string. Its purpose is to do nothing, but allow there to be a while loop. All the action of the while loop takes place inside its condition.

        Caution: Contents may have been coded under pressure.

        Thanks for the help everyone, I've also found you can write it like this as well:

        while ( <code goes here> ) {}

        The important but being that there's nothing in the {}. This gives us the original one-liner broken up to this:

        #!/usr/bin/perl my $string = 'abc abc abc \textit{abc abc} abc \textit{abc abc \textbf +{abc \textbf{abc abc} abc} abc} abc.'; while ($string =~ s/\\text(i|b)[tf]\{([^{}]*)\}/<$1>$2<\/$1>/g) {} print $string;

        Or you could take out the {} and add a 0, X, or whatever in front of the while (...)

        Condition for while, another form would be:

        '1 while' or 'A while' or ...