script fixstyle from cookbook

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

Hello,

2 questions on below example from Perl cookbook,

1)Never seen while being used inside of qutation while being assign to $code(on line 11) In this case, what is $code in line 17?

2)do not understand this line s{\\Q$in\\E}{$out}g at line 17
I looked up \Q and see that it means Quote metacharacters till \E
What does "quoting metacharacters" mean? I understand what metacharacter is(special character such as $ or ^) but just what is quoting metachracter mean??

\Q -- Quote (de-meta) metacharacters till \E.

 1 #!/usr/bin/perl -w
      2 # fixstyle - switch first set of <DATA> strings to second set
      3 #   usage: $0 [-v] [files ...]
      4 use strict;
      5 my $verbose = (@ARGV && $ARGV[0] eq '-v' && shift);
      6 if (@ARGV) {
      7   $^I = ".orig";          # preserve old files
      8 } else {
      9   die warn "$0: Reading from stdin\n" if -t STDIN;
     10 }
     11 my $code = "while (<>) {\n";
     12 # read in config, build up code to eval
     13 while (<DATA>) {
     14   chomp;
     15   my ($in, $out) = split /\s*=>\s*/;
     16   next unless $in && $out;
     17   $code .= "s{\\Q$in\\E}{$out}g";
     18   $code .= "&& printf STDERR qq($in => $out at \$ARGV line \$.
+\\n)"
     19                                                       if $verb
+ose;
     20   $code .= ";\n";
     21 }
     22 $code .= "print;\n}\n";
     23 eval "{ $code } 1" || die;
     24 __END__
     25 analysed        => analyzed
     26 built-in        => builtin
     27 chastized       => chastised
     28 commandline     => command-line
     29 de-allocate     => deallocate
     30 dropin          => drop-in
     31 hardcode        => hard-code
     32 meta-data       => metadata
     33 multicharacter  => multi-character
     34 multiway        => multi-way
     35 non-empty       => nonempty
     36 non-profit      => nonprofit
     37 non-trappable   => nontrappable
     38 pre-define      => predefine
     39 preextend       => pre-extend
     40 re-compiling    => recompiling
     41 reenter         => re-enter
     42 turnkey         => turn-key
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Comment on script fixstyle from cookbook Download Code

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Re: script fixstyle from cookbook by ysth (Canon) on Nov 08, 2007 at 07:03 UTC
Probably the easiest way to get a grip on what that code is doing is to replace the `eval "{ $code } 1"` with a `print "Code is: $code.\n";`, then look at the output and compare it to the lines that are generating it.	[reply] [d/l] [select]
Re: script fixstyle from cookbook by erroneousBollock (Curate) on Nov 08, 2007 at 06:30 UTC
Never seen while being used inside of qutation while being assign to $code(on line 11) while is not being used "inside" a quotation... $code is just a string being used to build up some code which will be evaluated with eval. In this case, what is $code in line 17? Depends on what's in the __DATA__ segment. What does "quoting metacharacters" mean? I understand what metacharacter is(special character such as $ or ^) but just what is quoting metachracter mean?? Metacharacters in this example are characters which have special meaning in a regular expression, such as `] [ } { ) ( ^ $`. See regular expressions. Since the loop is building code to be evaluated, it needs to escape regexp metacharacters so that they'll be correct after the `eval`. -David	[reply] [d/l] [select]
Re: script fixstyle from cookbook by cdarke (Prior) on Nov 08, 2007 at 11:45 UTC
What does "quoting metacharacters" mean? You will find that people also use the term 'escape', which can be equally baffling. Some characters have special meanings, as you are aware in regular expressions these are called 'meta-characters'. Within Perl (and languages such as UNIX shells) placing single quotes around a character removes its special meaning, such as ''. For single characters this is rather tedious, so placing a back-slash \ character in-front has the same effect, such as \. The Perl Q relieves the tedium further by placing a \ in front of each non-alphanumeric (includes _) character for us. In the olden days we sometimes needed character pairs which were prefixed with the <ESC> character, and this is how it was done, hence the term 'escape'. You will note that the \ character removes the special meaning from a special (meta) character, but can also add a special meaning to an ordinary character, for example "\n". It has to be inside double-quotes or qq (interpolation) for that to happen.	[reply]