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1 | =head1 NAME |
2 | ||
3 | perlfilter - Source Filters | |
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5 | =head1 DESCRIPTION |
6 | ||
7 | This article is about a little-known feature of Perl called | |
8 | I<source filters>. Source filters alter the program text of a module | |
9 | before Perl sees it, much as a C preprocessor alters the source text of | |
10 | a C program before the compiler sees it. This article tells you more | |
11 | about what source filters are, how they work, and how to write your | |
12 | own. | |
13 | ||
14 | The original purpose of source filters was to let you encrypt your | |
15 | program source to prevent casual piracy. This isn't all they can do, as | |
16 | you'll soon learn. But first, the basics. | |
17 | ||
18 | =head1 CONCEPTS | |
19 | ||
20 | Before the Perl interpreter can execute a Perl script, it must first | |
4449c45d GS |
21 | read it from a file into memory for parsing and compilation. If that |
22 | script itself includes other scripts with a C<use> or C<require> | |
23 | statement, then each of those scripts will have to be read from their | |
24 | respective files as well. | |
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25 | |
26 | Now think of each logical connection between the Perl parser and an | |
27 | individual file as a I<source stream>. A source stream is created when | |
28 | the Perl parser opens a file, it continues to exist as the source code | |
29 | is read into memory, and it is destroyed when Perl is finished parsing | |
30 | the file. If the parser encounters a C<require> or C<use> statement in | |
31 | a source stream, a new and distinct stream is created just for that | |
32 | file. | |
33 | ||
34 | The diagram below represents a single source stream, with the flow of | |
35 | source from a Perl script file on the left into the Perl parser on the | |
36 | right. This is how Perl normally operates. | |
37 | ||
38 | file -------> parser | |
39 | ||
40 | There are two important points to remember: | |
41 | ||
42 | =over 5 | |
43 | ||
44 | =item 1. | |
45 | ||
46 | Although there can be any number of source streams in existence at any | |
47 | given time, only one will be active. | |
48 | ||
49 | =item 2. | |
50 | ||
51 | Every source stream is associated with only one file. | |
52 | ||
53 | =back | |
54 | ||
55 | A source filter is a special kind of Perl module that intercepts and | |
56 | modifies a source stream before it reaches the parser. A source filter | |
40b7eeef | 57 | changes our diagram like this: |
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58 | |
59 | file ----> filter ----> parser | |
60 | ||
61 | If that doesn't make much sense, consider the analogy of a command | |
62 | pipeline. Say you have a shell script stored in the compressed file | |
63 | I<trial.gz>. The simple pipeline command below runs the script without | |
64 | needing to create a temporary file to hold the uncompressed file. | |
65 | ||
66 | gunzip -c trial.gz | sh | |
67 | ||
68 | In this case, the data flow from the pipeline can be represented as follows: | |
69 | ||
70 | trial.gz ----> gunzip ----> sh | |
71 | ||
72 | With source filters, you can store the text of your script compressed and use a source filter to uncompress it for Perl's parser: | |
73 | ||
74 | compressed gunzip | |
75 | Perl program ---> source filter ---> parser | |
76 | ||
77 | =head1 USING FILTERS | |
78 | ||
79 | So how do you use a source filter in a Perl script? Above, I said that | |
80 | a source filter is just a special kind of module. Like all Perl | |
81 | modules, a source filter is invoked with a use statement. | |
82 | ||
83 | Say you want to pass your Perl source through the C preprocessor before | |
84 | execution. You could use the existing C<-P> command line option to do | |
85 | this, but as it happens, the source filters distribution comes with a C | |
86 | preprocessor filter module called Filter::cpp. Let's use that instead. | |
87 | ||
88 | Below is an example program, C<cpp_test>, which makes use of this filter. | |
89 | Line numbers have been added to allow specific lines to be referenced | |
90 | easily. | |
91 | ||
92 | 1: use Filter::cpp ; | |
93 | 2: #define TRUE 1 | |
94 | 3: $a = TRUE ; | |
95 | 4: print "a = $a\n" ; | |
96 | ||
97 | When you execute this script, Perl creates a source stream for the | |
98 | file. Before the parser processes any of the lines from the file, the | |
99 | source stream looks like this: | |
100 | ||
101 | cpp_test ---------> parser | |
102 | ||
103 | Line 1, C<use Filter::cpp>, includes and installs the C<cpp> filter | |
104 | module. All source filters work this way. The use statement is compiled | |
105 | and executed at compile time, before any more of the file is read, and | |
106 | it attaches the cpp filter to the source stream behind the scenes. Now | |
107 | the data flow looks like this: | |
108 | ||
109 | cpp_test ----> cpp filter ----> parser | |
110 | ||
111 | As the parser reads the second and subsequent lines from the source | |
112 | stream, it feeds those lines through the C<cpp> source filter before | |
113 | processing them. The C<cpp> filter simply passes each line through the | |
114 | real C preprocessor. The output from the C preprocessor is then | |
115 | inserted back into the source stream by the filter. | |
116 | ||
117 | .-> cpp --. | |
118 | | | | |
119 | | | | |
120 | | <-' | |
121 | cpp_test ----> cpp filter ----> parser | |
122 | ||
123 | The parser then sees the following code: | |
124 | ||
125 | use Filter::cpp ; | |
126 | $a = 1 ; | |
127 | print "a = $a\n" ; | |
128 | ||
129 | Let's consider what happens when the filtered code includes another | |
130 | module with use: | |
131 | ||
132 | 1: use Filter::cpp ; | |
133 | 2: #define TRUE 1 | |
134 | 3: use Fred ; | |
135 | 4: $a = TRUE ; | |
136 | 5: print "a = $a\n" ; | |
137 | ||
138 | The C<cpp> filter does not apply to the text of the Fred module, only | |
139 | to the text of the file that used it (C<cpp_test>). Although the use | |
140 | statement on line 3 will pass through the cpp filter, the module that | |
141 | gets included (C<Fred>) will not. The source streams look like this | |
142 | after line 3 has been parsed and before line 4 is parsed: | |
143 | ||
144 | cpp_test ---> cpp filter ---> parser (INACTIVE) | |
145 | ||
146 | Fred.pm ----> parser | |
147 | ||
148 | As you can see, a new stream has been created for reading the source | |
149 | from C<Fred.pm>. This stream will remain active until all of C<Fred.pm> | |
150 | has been parsed. The source stream for C<cpp_test> will still exist, | |
151 | but is inactive. Once the parser has finished reading Fred.pm, the | |
152 | source stream associated with it will be destroyed. The source stream | |
153 | for C<cpp_test> then becomes active again and the parser reads line 4 | |
154 | and subsequent lines from C<cpp_test>. | |
155 | ||
156 | You can use more than one source filter on a single file. Similarly, | |
157 | you can reuse the same filter in as many files as you like. | |
158 | ||
159 | For example, if you have a uuencoded and compressed source file, it is | |
160 | possible to stack a uudecode filter and an uncompression filter like | |
161 | this: | |
162 | ||
163 | use Filter::uudecode ; use Filter::uncompress ; | |
164 | M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/ | |
165 | M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[> | |
166 | ... | |
167 | ||
168 | Once the first line has been processed, the flow will look like this: | |
169 | ||
170 | file ---> uudecode ---> uncompress ---> parser | |
171 | filter filter | |
172 | ||
173 | Data flows through filters in the same order they appear in the source | |
174 | file. The uudecode filter appeared before the uncompress filter, so the | |
175 | source file will be uudecoded before it's uncompressed. | |
176 | ||
177 | =head1 WRITING A SOURCE FILTER | |
178 | ||
179 | There are three ways to write your own source filter. You can write it | |
180 | in C, use an external program as a filter, or write the filter in Perl. | |
181 | I won't cover the first two in any great detail, so I'll get them out | |
182 | of the way first. Writing the filter in Perl is most convenient, so | |
183 | I'll devote the most space to it. | |
184 | ||
185 | =head1 WRITING A SOURCE FILTER IN C | |
186 | ||
187 | The first of the three available techniques is to write the filter | |
188 | completely in C. The external module you create interfaces directly | |
189 | with the source filter hooks provided by Perl. | |
190 | ||
191 | The advantage of this technique is that you have complete control over | |
192 | the implementation of your filter. The big disadvantage is the | |
193 | increased complexity required to write the filter - not only do you | |
194 | need to understand the source filter hooks, but you also need a | |
195 | reasonable knowledge of Perl guts. One of the few times it is worth | |
196 | going to this trouble is when writing a source scrambler. The | |
197 | C<decrypt> filter (which unscrambles the source before Perl parses it) | |
198 | included with the source filter distribution is an example of a C | |
199 | source filter (see Decryption Filters, below). | |
c47ff5f1 | 200 | |
c7c04614 GS |
201 | |
202 | =over 5 | |
203 | ||
204 | =item B<Decryption Filters> | |
205 | ||
206 | All decryption filters work on the principle of "security through | |
207 | obscurity." Regardless of how well you write a decryption filter and | |
208 | how strong your encryption algorithm, anyone determined enough can | |
209 | retrieve the original source code. The reason is quite simple - once | |
210 | the decryption filter has decrypted the source back to its original | |
211 | form, fragments of it will be stored in the computer's memory as Perl | |
212 | parses it. The source might only be in memory for a short period of | |
213 | time, but anyone possessing a debugger, skill, and lots of patience can | |
214 | eventually reconstruct your program. | |
215 | ||
216 | That said, there are a number of steps that can be taken to make life | |
217 | difficult for the potential cracker. The most important: Write your | |
218 | decryption filter in C and statically link the decryption module into | |
219 | the Perl binary. For further tips to make life difficult for the | |
220 | potential cracker, see the file I<decrypt.pm> in the source filters | |
221 | module. | |
222 | ||
223 | =back | |
224 | ||
225 | =head1 CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE | |
226 | ||
227 | An alternative to writing the filter in C is to create a separate | |
228 | executable in the language of your choice. The separate executable | |
229 | reads from standard input, does whatever processing is necessary, and | |
230 | writes the filtered data to standard output. C<Filter:cpp> is an | |
231 | example of a source filter implemented as a separate executable - the | |
232 | executable is the C preprocessor bundled with your C compiler. | |
233 | ||
234 | The source filter distribution includes two modules that simplify this | |
235 | task: C<Filter::exec> and C<Filter::sh>. Both allow you to run any | |
236 | external executable. Both use a coprocess to control the flow of data | |
237 | into and out of the external executable. (For details on coprocesses, | |
238 | see Stephens, W.R. "Advanced Programming in the UNIX Environment." | |
239 | Addison-Wesley, ISBN 0-210-56317-7, pages 441-445.) The difference | |
240 | between them is that C<Filter::exec> spawns the external command | |
241 | directly, while C<Filter::sh> spawns a shell to execute the external | |
242 | command. (Unix uses the Bourne shell; NT uses the cmd shell.) Spawning | |
243 | a shell allows you to make use of the shell metacharacters and | |
244 | redirection facilities. | |
245 | ||
246 | Here is an example script that uses C<Filter::sh>: | |
247 | ||
248 | use Filter::sh 'tr XYZ PQR' ; | |
249 | $a = 1 ; | |
250 | print "XYZ a = $a\n" ; | |
251 | ||
252 | The output you'll get when the script is executed: | |
253 | ||
254 | PQR a = 1 | |
255 | ||
256 | Writing a source filter as a separate executable works fine, but a | |
257 | small performance penalty is incurred. For example, if you execute the | |
258 | small example above, a separate subprocess will be created to run the | |
259 | Unix C<tr> command. Each use of the filter requires its own subprocess. | |
260 | If creating subprocesses is expensive on your system, you might want to | |
261 | consider one of the other options for creating source filters. | |
262 | ||
263 | =head1 WRITING A SOURCE FILTER IN PERL | |
264 | ||
265 | The easiest and most portable option available for creating your own | |
266 | source filter is to write it completely in Perl. To distinguish this | |
267 | from the previous two techniques, I'll call it a Perl source filter. | |
268 | ||
269 | To help understand how to write a Perl source filter we need an example | |
270 | to study. Here is a complete source filter that performs rot13 | |
271 | decoding. (Rot13 is a very simple encryption scheme used in Usenet | |
272 | postings to hide the contents of offensive posts. It moves every letter | |
273 | forward thirteen places, so that A becomes N, B becomes O, and Z | |
274 | becomes M.) | |
275 | ||
276 | ||
277 | package Rot13 ; | |
278 | ||
279 | use Filter::Util::Call ; | |
280 | ||
281 | sub import { | |
282 | my ($type) = @_ ; | |
283 | my ($ref) = [] ; | |
284 | filter_add(bless $ref) ; | |
285 | } | |
286 | ||
287 | sub filter { | |
288 | my ($self) = @_ ; | |
289 | my ($status) ; | |
290 | ||
291 | tr/n-za-mN-ZA-M/a-zA-Z/ | |
292 | if ($status = filter_read()) > 0 ; | |
293 | $status ; | |
294 | } | |
295 | ||
296 | 1; | |
297 | ||
298 | All Perl source filters are implemented as Perl classes and have the | |
299 | same basic structure as the example above. | |
300 | ||
301 | First, we include the C<Filter::Util::Call> module, which exports a | |
302 | number of functions into your filter's namespace. The filter shown | |
303 | above uses two of these functions, C<filter_add()> and | |
304 | C<filter_read()>. | |
305 | ||
306 | Next, we create the filter object and associate it with the source | |
307 | stream by defining the C<import> function. If you know Perl well | |
308 | enough, you know that C<import> is called automatically every time a | |
309 | module is included with a use statement. This makes C<import> the ideal | |
310 | place to both create and install a filter object. | |
311 | ||
312 | In the example filter, the object (C<$ref>) is blessed just like any | |
313 | other Perl object. Our example uses an anonymous array, but this isn't | |
314 | a requirement. Because this example doesn't need to store any context | |
315 | information, we could have used a scalar or hash reference just as | |
316 | well. The next section demonstrates context data. | |
317 | ||
318 | The association between the filter object and the source stream is made | |
319 | with the C<filter_add()> function. This takes a filter object as a | |
320 | parameter (C<$ref> in this case) and installs it in the source stream. | |
321 | ||
322 | Finally, there is the code that actually does the filtering. For this | |
323 | type of Perl source filter, all the filtering is done in a method | |
324 | called C<filter()>. (It is also possible to write a Perl source filter | |
325 | using a closure. See the C<Filter::Util::Call> manual page for more | |
326 | details.) It's called every time the Perl parser needs another line of | |
327 | source to process. The C<filter()> method, in turn, reads lines from | |
328 | the source stream using the C<filter_read()> function. | |
329 | ||
330 | If a line was available from the source stream, C<filter_read()> | |
331 | returns a status value greater than zero and appends the line to C<$_>. | |
332 | A status value of zero indicates end-of-file, less than zero means an | |
333 | error. The filter function itself is expected to return its status in | |
334 | the same way, and put the filtered line it wants written to the source | |
335 | stream in C<$_>. The use of C<$_> accounts for the brevity of most Perl | |
336 | source filters. | |
337 | ||
338 | In order to make use of the rot13 filter we need some way of encoding | |
339 | the source file in rot13 format. The script below, C<mkrot13>, does | |
340 | just that. | |
341 | ||
342 | die "usage mkrot13 filename\n" unless @ARGV ; | |
343 | my $in = $ARGV[0] ; | |
344 | my $out = "$in.tmp" ; | |
345 | open(IN, "<$in") or die "Cannot open file $in: $!\n"; | |
346 | open(OUT, ">$out") or die "Cannot open file $out: $!\n"; | |
347 | ||
348 | print OUT "use Rot13;\n" ; | |
349 | while (<IN>) { | |
350 | tr/a-zA-Z/n-za-mN-ZA-M/ ; | |
351 | print OUT ; | |
352 | } | |
353 | ||
354 | close IN; | |
355 | close OUT; | |
356 | unlink $in; | |
357 | rename $out, $in; | |
358 | ||
359 | If we encrypt this with C<mkrot13>: | |
360 | ||
361 | print " hello fred \n" ; | |
362 | ||
363 | the result will be this: | |
364 | ||
365 | use Rot13; | |
366 | cevag "uryyb serq\a" ; | |
367 | ||
368 | Running it produces this output: | |
369 | ||
370 | hello fred | |
371 | ||
372 | =head1 USING CONTEXT: THE DEBUG FILTER | |
373 | ||
374 | The rot13 example was a trivial example. Here's another demonstration | |
375 | that shows off a few more features. | |
376 | ||
377 | Say you wanted to include a lot of debugging code in your Perl script | |
378 | during development, but you didn't want it available in the released | |
379 | product. Source filters offer a solution. In order to keep the example | |
380 | simple, let's say you wanted the debugging output to be controlled by | |
381 | an environment variable, C<DEBUG>. Debugging code is enabled if the | |
382 | variable exists, otherwise it is disabled. | |
383 | ||
384 | Two special marker lines will bracket debugging code, like this: | |
385 | ||
386 | ## DEBUG_BEGIN | |
387 | if ($year > 1999) { | |
388 | warn "Debug: millennium bug in year $year\n" ; | |
389 | } | |
390 | ## DEBUG_END | |
391 | ||
392 | When the C<DEBUG> environment variable exists, the filter ensures that | |
393 | Perl parses only the code between the C<DEBUG_BEGIN> and C<DEBUG_END> | |
394 | markers. That means that when C<DEBUG> does exist, the code above | |
395 | should be passed through the filter unchanged. The marker lines can | |
396 | also be passed through as-is, because the Perl parser will see them as | |
397 | comment lines. When C<DEBUG> isn't set, we need a way to disable the | |
398 | debug code. A simple way to achieve that is to convert the lines | |
399 | between the two markers into comments: | |
400 | ||
401 | ## DEBUG_BEGIN | |
402 | #if ($year > 1999) { | |
403 | # warn "Debug: millennium bug in year $year\n" ; | |
404 | #} | |
405 | ## DEBUG_END | |
406 | ||
407 | Here is the complete Debug filter: | |
408 | ||
409 | package Debug; | |
410 | ||
411 | use strict; | |
9f1b1f2d | 412 | use warnings; |
c7c04614 GS |
413 | use Filter::Util::Call ; |
414 | ||
415 | use constant TRUE => 1 ; | |
416 | use constant FALSE => 0 ; | |
417 | ||
418 | sub import { | |
419 | my ($type) = @_ ; | |
420 | my (%context) = ( | |
421 | Enabled => defined $ENV{DEBUG}, | |
422 | InTraceBlock => FALSE, | |
423 | Filename => (caller)[1], | |
424 | LineNo => 0, | |
425 | LastBegin => 0, | |
426 | ) ; | |
427 | filter_add(bless \%context) ; | |
428 | } | |
429 | ||
430 | sub Die { | |
431 | my ($self) = shift ; | |
432 | my ($message) = shift ; | |
433 | my ($line_no) = shift || $self->{LastBegin} ; | |
434 | die "$message at $self->{Filename} line $line_no.\n" | |
435 | } | |
436 | ||
437 | sub filter { | |
438 | my ($self) = @_ ; | |
439 | my ($status) ; | |
440 | $status = filter_read() ; | |
441 | ++ $self->{LineNo} ; | |
442 | ||
443 | # deal with EOF/error first | |
444 | if ($status <= 0) { | |
445 | $self->Die("DEBUG_BEGIN has no DEBUG_END") | |
446 | if $self->{InTraceBlock} ; | |
447 | return $status ; | |
448 | } | |
449 | ||
450 | if ($self->{InTraceBlock}) { | |
451 | if (/^\s*##\s*DEBUG_BEGIN/ ) { | |
452 | $self->Die("Nested DEBUG_BEGIN", $self->{LineNo}) | |
453 | } elsif (/^\s*##\s*DEBUG_END/) { | |
454 | $self->{InTraceBlock} = FALSE ; | |
455 | } | |
456 | ||
457 | # comment out the debug lines when the filter is disabled | |
458 | s/^/#/ if ! $self->{Enabled} ; | |
459 | } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) { | |
460 | $self->{InTraceBlock} = TRUE ; | |
461 | $self->{LastBegin} = $self->{LineNo} ; | |
462 | } elsif ( /^\s*##\s*DEBUG_END/ ) { | |
463 | $self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo}); | |
464 | } | |
465 | return $status ; | |
466 | } | |
467 | ||
468 | 1 ; | |
469 | ||
470 | The big difference between this filter and the previous example is the | |
471 | use of context data in the filter object. The filter object is based on | |
472 | a hash reference, and is used to keep various pieces of context | |
473 | information between calls to the filter function. All but two of the | |
474 | hash fields are used for error reporting. The first of those two, | |
475 | Enabled, is used by the filter to determine whether the debugging code | |
476 | should be given to the Perl parser. The second, InTraceBlock, is true | |
477 | when the filter has encountered a C<DEBUG_BEGIN> line, but has not yet | |
478 | encountered the following C<DEBUG_END> line. | |
479 | ||
480 | If you ignore all the error checking that most of the code does, the | |
481 | essence of the filter is as follows: | |
482 | ||
483 | sub filter { | |
484 | my ($self) = @_ ; | |
485 | my ($status) ; | |
486 | $status = filter_read() ; | |
487 | ||
488 | # deal with EOF/error first | |
489 | return $status if $status <= 0 ; | |
490 | if ($self->{InTraceBlock}) { | |
491 | if (/^\s*##\s*DEBUG_END/) { | |
492 | $self->{InTraceBlock} = FALSE | |
493 | } | |
494 | ||
495 | # comment out debug lines when the filter is disabled | |
496 | s/^/#/ if ! $self->{Enabled} ; | |
497 | } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) { | |
498 | $self->{InTraceBlock} = TRUE ; | |
499 | } | |
500 | return $status ; | |
501 | } | |
502 | ||
503 | Be warned: just as the C-preprocessor doesn't know C, the Debug filter | |
504 | doesn't know Perl. It can be fooled quite easily: | |
505 | ||
506 | print <<EOM; | |
507 | ##DEBUG_BEGIN | |
508 | EOM | |
509 | ||
510 | Such things aside, you can see that a lot can be achieved with a modest | |
40b7eeef | 511 | amount of code. |
c7c04614 GS |
512 | |
513 | =head1 CONCLUSION | |
514 | ||
515 | You now have better understanding of what a source filter is, and you | |
516 | might even have a possible use for them. If you feel like playing with | |
517 | source filters but need a bit of inspiration, here are some extra | |
518 | features you could add to the Debug filter. | |
519 | ||
520 | First, an easy one. Rather than having debugging code that is | |
521 | all-or-nothing, it would be much more useful to be able to control | |
522 | which specific blocks of debugging code get included. Try extending the | |
523 | syntax for debug blocks to allow each to be identified. The contents of | |
524 | the C<DEBUG> environment variable can then be used to control which | |
525 | blocks get included. | |
526 | ||
527 | Once you can identify individual blocks, try allowing them to be | |
528 | nested. That isn't difficult either. | |
529 | ||
d1be9408 | 530 | Here is an interesting idea that doesn't involve the Debug filter. |
c7c04614 GS |
531 | Currently Perl subroutines have fairly limited support for formal |
532 | parameter lists. You can specify the number of parameters and their | |
533 | type, but you still have to manually take them out of the C<@_> array | |
534 | yourself. Write a source filter that allows you to have a named | |
535 | parameter list. Such a filter would turn this: | |
536 | ||
537 | sub MySub ($first, $second, @rest) { ... } | |
538 | ||
539 | into this: | |
540 | ||
541 | sub MySub($$@) { | |
542 | my ($first) = shift ; | |
543 | my ($second) = shift ; | |
544 | my (@rest) = @_ ; | |
545 | ... | |
546 | } | |
547 | ||
548 | Finally, if you feel like a real challenge, have a go at writing a | |
549 | full-blown Perl macro preprocessor as a source filter. Borrow the | |
550 | useful features from the C preprocessor and any other macro processors | |
551 | you know. The tricky bit will be choosing how much knowledge of Perl's | |
552 | syntax you want your filter to have. | |
553 | ||
554 | =head1 REQUIREMENTS | |
555 | ||
556 | The Source Filters distribution is available on CPAN, in | |
557 | ||
558 | CPAN/modules/by-module/Filter | |
559 | ||
83df6a1d JH |
560 | Starting from Perl 5.8 Filter::Util::Call (the core part of the |
561 | Source Filters distribution) is part of the standard Perl distribution. | |
562 | Also included is a friendlier interface called Filter::Simple, by | |
563 | Damian Conway. | |
564 | ||
c7c04614 GS |
565 | =head1 AUTHOR |
566 | ||
567 | Paul Marquess E<lt>Paul.Marquess@btinternet.comE<gt> | |
568 | ||
569 | =head1 Copyrights | |
570 | ||
571 | This article originally appeared in The Perl Journal #11, and is | |
572 | copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and | |
573 | The Perl Journal. This document may be distributed under the same terms | |
574 | as Perl itself. |