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15.5.18.6 Valid Uses Of String Interpolation

Perl is often used to generate sources for other programming languages or arbitrary file formats. Web applications that output HTML code make a prominent example for such usage.

You will often come across situations where you want to intersperse code written in the target (programming) language with translatable messages, like in the following HTML example:

     print gettext <<EOF;
     <h1>My Homepage</h1>
     <script language="JavaScript"><!--
     for (i = 0; i < 100; ++i) {
         alert ("Thank you so much for visiting my homepage!");
     }
     //--></script>
     EOF

The parser will extract the entire here document, and it will appear entirely in the resulting PO file, including the JavaScript snippet embedded in the HTML code. If you exaggerate with constructs like the above, you will run the risk that the translators of your package will look out for a less challenging project. You should consider an alternative expression here:

     print <<EOF;
     <h1>$gettext{"My Homepage"}</h1>
     <script language="JavaScript"><!--
     for (i = 0; i < 100; ++i) {
         alert ("$gettext{'Thank you so much for visiting my homepage!'}");
     }
     //--></script>
     EOF

Only the translatable portions of the code will be extracted here, and the resulting PO file will begrudgingly improve in terms of readability.

You can interpolate hash lookups in all strings or quote-like expressions that are subject to interpolation (see the manual page ‘man perlop’ for details). Double interpolation is invalid, however:

     # TRANSLATORS: Replace "the earth" with the name of your planet.
     print gettext qq{Welcome to $gettext->{"the earth"}};

The qq-quoted string is recognized as an argument to xgettext in the first place, and checked for invalid variable interpolation. The dollar sign of hash-dereferencing will therefore terminate the parser with an “invalid interpolation” error.

It is valid to interpolate hash lookups in regular expressions:

     if ($var =~ /$gettext{"the earth"}/) {
        print gettext "Match!\n";
     }
     s/$gettext{"U. S. A."}/$gettext{"U. S. A."} $gettext{"(dial +0)"}/g;