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 15.5.18.1 General Problems Parsing Perl Code
 ............................................
 
 It is often heard that only Perl can parse Perl.  This is not true.
 Perl cannot be _parsed_ at all, it can only be _executed_.  Perl has
 various built-in ambiguities that can only be resolved at runtime.
 
    The following example may illustrate one common problem:
 
      print gettext "Hello World!";
 
    Although this example looks like a bullet-proof case of a function
 invocation, it is not:
 
      open gettext, ">testfile" or die;
      print gettext "Hello world!"
 
    In this context, the string `gettext' looks more like a file handle.
 But not necessarily:
 
      use Locale::Messages qw (:libintl_h);
      open gettext ">testfile" or die;
      print gettext "Hello world!";
 
    Now, the file is probably syntactically incorrect, provided that the
 module `Locale::Messages' found first in the Perl include path exports a
 function `gettext'.  But what if the module `Locale::Messages' really
 looks like this?
 
      use vars qw (*gettext);
 
      1;
 
    In this case, the string `gettext' will be interpreted as a file
 handle again, and the above example will create a file `testfile' and
 write the string "Hello world!" into it.  Even advanced control flow
 analysis will not really help:
 
      if (0.5 < rand) {
         eval "use Sane";
      } else {
         eval "use InSane";
      }
      print gettext "Hello world!";
 
    If the module `Sane' exports a function `gettext' that does what we
 expect, and the module `InSane' opens a file for writing and associates
 the _handle_ `gettext' with this output stream, we are clueless again
 about what will happen at runtime.  It is completely unpredictable.
 The truth is that Perl has so many ways to fill its symbol table at
 runtime that it is impossible to interpret a particular piece of code
 without executing it.
 
    Of course, `xgettext' will not execute your Perl sources while
 scanning for translatable strings, but rather use heuristics in order
 to guess what you meant.
 
    Another problem is the ambiguity of the slash and the question mark.
 Their interpretation depends on the context:
 
      # A pattern match.
      print "OK\n" if /foobar/;
 
      # A division.
      print 1 / 2;
 
      # Another pattern match.
      print "OK\n" if ?foobar?;
 
      # Conditional.
      print $x ? "foo" : "bar";
 
    The slash may either act as the division operator or introduce a
 pattern match, whereas the question mark may act as the ternary
 conditional operator or as a pattern match, too.  Other programming
 languages like `awk' present similar problems, but the consequences of a
 misinterpretation are particularly nasty with Perl sources.  In `awk'
 for instance, a statement can never exceed one line and the parser can
 recover from a parsing error at the next newline and interpret the rest
 of the input stream correctly.  Perl is different, as a pattern match
 is terminated by the next appearance of the delimiter (the slash or the
 question mark) in the input stream, regardless of the semantic context.
 If a slash is really a division sign but mis-interpreted as a pattern
 match, the rest of the input file is most probably parsed incorrectly.
 
    If you find that `xgettext' fails to extract strings from portions
 of your sources, you should therefore look out for slashes and/or
 question marks preceding these sections.  You may have come across a
 bug in `xgettext''s Perl parser (and of course you should report that
 bug).  In the meantime you should consider to reformulate your code in
 a manner less challenging to `xgettext'.
 

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