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Perl Style: Avoid Byte Processing

  • C programmers often try to process strings a byte at a time. Don’t do that! Perl makes it easy to take data in big bites.
  • Don’t use getc. Grab the whole line and operate on it all at once.
  • Even operations traditionally done a char at a time in C, like lexing, should be done differently. For example:

        @chars = split //, $input;
        while (@chars) {
          $c = shift @chars;
          # State machine;
        }
    

    Is far too low level. Try something more like:

        sub parse_expr {
        local $_ = shift;
        my @tokens = ();
        my $paren = 0;
        my $want_term = 1;
    
        while (length) {
            s/^\s*//;
    
            if (s/^\(//) {
            return unless $want_term;
            push @tokens, '(';
            $paren++;
            $want_term = 1;
            next;
            } 
    
            if (s/^\)//) {
            return if $want_term;
            push @tokens, ')';
            if ($paren < 1) {
                return;
            } 
            --$paren;
            $want_term = 0;
            next;
            } 
    
            if (s/^and\b//i || s/^&&?//) {
            return if $want_term;
            push @tokens, '&';
            $want_term = 1;
            next;
            } 
    
            if (s/^or\b//i || s/^\|\|?//) {
            return if $want_term;
            push @tokens, '|';
            $want_term = 1;
            next;
            } 
    
            if (s/^not\b//i || s/^~// || s/^!//) {
            return unless $want_term;
            push @tokens, '~';
            $want_term = 1;
            next;
            } 
    
            if (s/^(\w+)//) {
            push @tokens, '&' unless $want_term;
            push @tokens, $1 . '()';
            $want_term = 0;
            next;
            } 
    
            return;
    
        }
        return "@tokens";
        }
    

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Copyright © 1998, Tom Christiansen All rights reserved.

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