Perl Style: Avoid Byte Processing

Jan 1, 1998 by Tom Christiansen

• 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.