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(gettext) Interpolation I

Info Catalog (gettext) Quote-like Expressions (gettext) Perl (gettext) Interpolation II
 
 15.5.18.5 Invalid Uses Of String Interpolation
 ..............................................
 
 Perl is capable of interpolating variables into strings.  This offers
 some nice features in localized programs but can also lead to problems.
 
    A common error is a construct like the following:
 
      print gettext "This is the program $0!\n";
 
    Perl will interpolate at runtime the value of the variable `$0' into
 the argument of the `gettext()' function.  Hence, this argument is not
 a string constant but a variable argument (`$0' is a global variable
 that holds the name of the Perl script being executed).  The
 interpolation is performed by Perl before the string argument is passed
 to `gettext()' and will therefore depend on the name of the script
 which can only be determined at runtime.  Consequently, it is almost
 impossible that a translation can be looked up at runtime (except if,
 by accident, the interpolated string is found in the message catalog).
 
    The `xgettext' program will therefore terminate parsing with a fatal
 error if it encounters a variable inside of an extracted string.  In
 general, this will happen for all kinds of string interpolations that
 cannot be safely performed at compile time.  If you absolutely know
 what you are doing, you can always circumvent this behavior:
 
      my $know_what_i_am_doing = "This is program $0!\n";
      print gettext $know_what_i_am_doing;
 
    Since the parser only recognizes strings and quote-like expressions,
 but not variables or other terms, the above construct will be accepted.
 You will have to find another way, however, to let your original
 string make it into your message catalog.
 
    If invoked with the option `--extract-all', resp. `-a', variable
 interpolation will be accepted.  Rationale: You will generally use this
 option in order to prepare your sources for internationalization.
 
    Please see the manual page `man perlop' for details of strings and
 quote-like expressions that are subject to interpolation and those that
 are not.  Safe interpolations (that will not lead to a fatal error) are:
 
    * the escape sequences `\t' (tab, HT, TAB), `\n' (newline, NL), `\r'
      (return, CR), `\f' (form feed, FF), `\b' (backspace, BS), `\a'
      (alarm, bell, BEL), and `\e' (escape, ESC).
 
    * octal chars, like `\033'
      Note that octal escapes in the range of 400-777 are translated
      into a UTF-8 representation, regardless of the presence of the
      `use utf8' pragma.
 
    * hex chars, like `\x1b'
 
    * wide hex chars, like `\x{263a}'
      Note that this escape is translated into a UTF-8 representation,
      regardless of the presence of the `use utf8' pragma.
 
    * control chars, like `\c[' (CTRL-[)
 
    * named Unicode chars, like `\N{LATIN CAPITAL LETTER C WITH CEDILLA}'
      Note that this escape is translated into a UTF-8 representation,
      regardless of the presence of the `use utf8' pragma.
 
    The following escapes are considered partially safe:
 
    * `\l' lowercase next char
 
    * `\u' uppercase next char
 
    * `\L' lowercase till \E
 
    * `\U' uppercase till \E
 
    * `\E' end case modification
 
    * `\Q' quote non-word characters till \E
 
 
    These escapes are only considered safe if the string consists of
 ASCII characters only.  Translation of characters outside the range
 defined by ASCII is locale-dependent and can actually only be performed
 at runtime; `xgettext' doesn't do these locale-dependent translations
 at extraction time.
 
    Except for the modifier `\Q', these translations, albeit valid, are
 generally useless and only obfuscate your sources.  If a translation
 can be safely performed at compile time you can just as well write what
 you mean.
 
Info Catalog (gettext) Quote-like Expressions (gettext) Perl (gettext) Interpolation II
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