ENC2XS(1) Perl Programmers Reference Guide ENC2XS(1)
enc2xs -- Perl Encode Module Generator
enc2xs -M ModName mapfiles...
enc2xs builds a Perl extension for use by Encode from either Unicode
Character Mapping files (.ucm) or Tcl Encoding Files (.enc). Besides
being used internally during the build process of the Encode module, you
can use enc2xs to add your own encoding to perl. No knowledge of XS is
If you want to know as little about Perl as possible but need to add a
new encoding, just read this chapter and forget the rest.
0. Have a .ucm file ready. You can get it from somewhere or you can
write your own from scratch or you can grab one from the Encode
distribution and customize it. For the UCM format, see the next
Chapter. In the example below, I'll call my theoretical encoding
myascii, defined in my.ucm. "$" is a shell prompt.
$ ls -F
1. Issue a command as follows;
$ enc2xs -M My my.ucm
Now take a look at your current directory. It should look like this.
$ ls -F
Makefile.PL My.pm my.ucm t/
The following files were created.
Makefile.PL - MakeMaker script
My.pm - Encode submodule
t/My.t - test file
If you want *.ucm installed together with the modules, do as
$ mkdir Encode
$ mv *.ucm Encode
$ enc2xs -M My Encode/*ucm
2. Edit the files generated. You don't have to if you have no time AND
no intention to give it to someone else. But it is a good idea to
edit the pod and to add more tests.
3. Now issue a command all Perl Mongers love:
$ perl Makefile.PL
Writing Makefile for Encode::My
4. Now all you have to do is make.
cp My.pm blib/lib/Encode/My.pm
/usr/local/bin/perl /usr/local/bin/enc2xs -Q -O \
-o encode_t.c -f encode_t.fnm
Reading myascii (myascii)
Writing compiled form
128 bytes in string tables
384 bytes (75%) saved spotting duplicates
1 bytes (0.775%) saved using substrings
chmod 644 blib/arch/auto/Encode/My/My.bs
The time it takes varies depending on how fast your machine is and
how large your encoding is. Unless you are working on something big
like euc-tw, it won't take too long.
5. You can "make install" already but you should test first.
$ make test
PERL_DL_NONLAZY=1 /usr/local/bin/perl -Iblib/arch -Iblib/lib \
-e 'use Test::Harness qw(&runtests $verbose); \
$verbose=0; runtests @ARGV;' t/*.t
All tests successful.
Files=1, Tests=2, 0 wallclock secs
( 0.09 cusr + 0.01 csys = 0.09 CPU)
6. If you are content with the test result, just "make install"
7. If you want to add your encoding to Encode's demand-loading list (so
you don't have to "use Encode::YourEncoding"), run
to update Encode::ConfigLocal, a module that controls local settings.
After that, "use Encode;" is enough to load your encodings on demand.
The Unicode Character Map
Encode uses the Unicode Character Map (UCM) format for source character
mappings. This format is used by IBM's ICU package and was adopted by
Nick Ing-Simmons for use with the Encode module. Since UCM is more
flexible than Tcl's Encoding Map and far more user-friendly, this is the
recommended format for Encode now.
A UCM file looks like this.
<code_set_name> "US-ascii" # Required
<code_set_alias> "ascii" # Optional
<mb_cur_min> 1 # Required; usually 1
<mb_cur_max> 1 # Max. # of bytes/char
<subchar> \x3F # Substitution char
<U0000> \x00 |0 # <control>
<U0001> \x01 |0 # <control>
<U0002> \x02 |0 # <control>
<U007C> \x7C |0 # VERTICAL LINE
<U007D> \x7D |0 # RIGHT CURLY BRACKET
<U007E> \x7E |0 # TILDE
<U007F> \x7F |0 # <control>
• Anything that follows "#" is treated as a comment.
• The header section continues until a line containing the word
CHARMAP. This section has a form of <keyword> value, one pair per
line. Strings used as values must be quoted. Barewords are treated
as numbers. \xXX represents a byte.
Most of the keywords are self-explanatory. subchar means substitution
character, not subcharacter. When you decode a Unicode sequence to
this encoding but no matching character is found, the byte sequence
defined here will be used. For most cases, the value here is \x3F;
in ASCII, this is a question mark.
• CHARMAP starts the character map section. Each line has a form as
<UXXXX> \xXX.. |0 # comment
^ ^ ^
| | +- Fallback flag
| +-------- Encoded byte sequence
+-------------- Unicode Character ID in hex
The format is roughly the same as a header section except for the
fallback flag: | followed by 0..3. The meaning of the possible
values is as follows:
|0 Round trip safe. A character decoded to Unicode encodes back to
the same byte sequence. Most characters have this flag.
|1 Fallback for unicode -> encoding. When seen, enc2xs adds this
character for the encode map only.
|2 Skip sub-char mapping should there be no code point.
|3 Fallback for encoding -> unicode. When seen, enc2xs adds this
character for the decode map only.
• And finally, END OF CHARMAP ends the section.
When you are manually creating a UCM file, you should copy ascii.ucm or
an existing encoding which is close to yours, rather than write your own
When you do so, make sure you leave at least U0000 to U0020 as is, unless
your environment is EBCDIC.
CAVEAT: not all features in UCM are implemented. For example, icu:state
is not used. Because of that, you need to write a perl module if you
want to support algorithmical encodings, notably the ISO-2022 series.
Such modules include Encode::JP::2022_JP, Encode::KR::2022_KR, and
Coping with duplicate mappings
When you create a map, you SHOULD make your mappings round-trip safe.
That is, "encode('your-encoding', decode('your-encoding', $data)) eq
$data" stands for all characters that are marked as "|0". Here is how to
• Sort your map in Unicode order.
• When you have a duplicate entry, mark either one with '|1' or '|3'.
• And make sure the '|1' or '|3' entry FOLLOWS the '|0' entry.
Here is an example from big5-eten.
<U2550> \xF9\xF9 |0
<U2550> \xA2\xA4 |3
Internally Encoding -> Unicode and Unicode -> Encoding Map looks like
E to U U to E
\xF9\xF9 => U2550 U2550 => \xF9\xF9
\xA2\xA4 => U2550
So it is round-trip safe for \xF9\xF9. But if the line above is upside
down, here is what happens.
E to U U to E
\xA2\xA4 => U2550 U2550 => \xF9\xF9
(\xF9\xF9 => U2550 is now overwritten!)
The Encode package comes with ucmlint, a crude but sufficient utility to
check the integrity of a UCM file. Check under the Encode/bin directory
When in doubt, you can use ucmsort, yet another utility under Encode/bin
• ICU Home Page <http://www.icu-project.org/>
• ICU Character Mapping Tables
• ICU:Conversion Data
Encode, perlmod, perlpod
perl v5.30.3 2021-11-13 ENC2XS(1)