[privoxy.git] / doc / pcrs.3

.\" Copyright (c) 2001 Andreas S. Oesterhelt <oes@oesterhelt.org>
.\"
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.TH PCRS 3 "17 August 2001"
.SH NAME
pcrs - Perl-compatible regular substitution.
.SH SYNOPSIS
.br
.BI "#include <pcrs.h>"
.PP
.br
.BI "pcrs_job *pcrs_compile(const char *" "pattern" "," 
.ti +5n
.BI "const char *" "substitute" ", const char *" "options" ","
.ti +5n
.BI "int *" "errptr" ");"
.PP
.br
.BI "pcrs_job *pcrs_compile_command(const char *" "command" ","
.ti +5n
.BI "int *" "errptr");
.PP
.br
.BI "int pcrs_execute(pcrs_job *" "job" ", char *" "subject" ","
.ti +5n
.BI "int " "subject_length" ", char **" "result" ","
.ti +5n
.BI "int *" "result_length" ");"
.PP
.br
.BI "int pcrs_execute_list (pcrs_job *" "joblist" ", char *" "subject" ","
.ti +5n
.BI "int " "subject_length" ", char **" "result" ","
.ti +5n
.BI "int *" "result_length" ");"
.PP
.br
.BI "pcrs_job *pcrs_free_job(pcrs_job *" "job" ");"
.PP
.br
.BI "void pcrs_free_joblist(pcrs_job *" "joblist" ");"
.PP
.br
.BI "char *pcrs_strerror(int " "err" ");"
.PP
.br

.SH DESCRIPTION
The
.SM PCRS
library is a supplement to the 
.SB PCRE(3)
library that implements
.RB "regular expression based substitution, like provided by " "Perl(1)" "'s 's'"
operator. It uses the same syntax and semantics as Perl 5, with just a few
differences (see below).

In a first step, the information on a substitution, i.e. the pattern, the
substitute and the options are compiled from Perl syntax to an internal form
.RB "called " "pcrs_job" " by using either the " "pcrs_compile()" " or " 
.BR "pcrs_compile_command()" " functions."

Once the job is compiled, it can be used on subjects, which are arbitrary 
memory areas containing string or binary data, by calling
.BR "pcrs_execute()" ". Jobs can be chained to joblists and whole"
.RB "joblists can be applied to a subject using " "pcrs_execute_list()" "."

There are also convenience functions for freeing the jobs and for errno-to-string
.RB "conversion, namely " "pcrs_free_job()" ", " "pcrs_free_joblist()" " and "
.BR "pcrs_strerror()" "."

.SH COMPILING JOBS
.RB "The function " "pcrs_compile()" " is called to compile a " "pcrs_job" 
.RI "from a " "pattern" ", " "substitute" " and " "options" " string.
.RB "The resulting " "pcrs_job" " structure is dynamically allocated and it"
.RB "is the caller's responsibility to " "free()" " it when it's no longer needed."

.BR "pcrs_compile_command()" " is a convenience wrapper function that parses a Perl"
.IR "command" " of the form"
.BI "s/" "pattern" "/" "substitute" "/[" "options" "]"
.RB "into its components and then calls " "pcrs_compile()" ". As in Perl, you"
.RB "are not bound to the '" "/" "' character: Whatever"
.RB "follows the '" "s" "' will be used as the delimiter. Patterns or substitutes"
that contain the delimiter need to quote it:
\fBs/th\\/is/th\\/at/\fR
.RB "will replace " "th/is" " by " "th/at" " and can be written more simply as" 
.BR "s|th/is|th/at|" "."

.IR "pattern" ", " "substitute" ", " "options" " and " "command" " must be"
.RI "zero-terminated C strings. " "substitute" " and " "options" " may be"
.BR "NULL" ", in which case they are treated like the empty string."

.SS "Return value and diagnostics"
On success, both functions return a pointer to the compiled job.
.RB "On failure, " "NULL"
.RI "is returned. In that case, the pcrs error code is written to *" "err" "."

.SS Patterns
.RI "For the syntax of the " "pattern" ", see the "
.BR "PCRE(3)" " manual page."

.SS Substitutes
.RI "The " "substitute" " uses"
.RB "Perl syntax as documented in the " "perlre(1)" " manual page, with"
some exceptions: 

Most notably and evidently, since
.SM PCRS
is not Perl, variable interpolation or Perl command substitution won't work.
Special variables that do get interpolated, are:
.TP
.B "$1, $2, ..., $n"
Like in Perl, these variables refer to what the nth capturing subpattern
in the pattern matched.
.TP
.B "$& and $0"
.RB "refer to the whole match. Note that " "$0" " is deprecated in recent"
Perl versions and now refers to the program name.
.TP
.B "$+"
refers to what the last capturing subpattern matched.
.TP
.BR "$` and $'" " (backtick and tick)"
.RI "refer to the areas of the " "subject" " before and after the match, respectively."
.RB "Note that, like in Perl, the " "unmodified" " subject is used, even"
if a global substitution previously matched.

.PP
Perl4-style references to subpattern matches of the form 
\fB\\1, \\2, ...\fR
.RB "which only exist in Perl5 for backwards compatibility, are " "not" 
supported.

Also, since the substitute is a double-quoted string in Perl, you
might expect all Perl syntax for special characters to apply. In fact,
only the following are supported:

.TP
\fB\\n\fR
newline (0x0a)
.TP
\fB\\r\fR
carriage return (0x0d)
.TP
\fB\\t\fR
horizontal tab (0x09)
.TP
\fB\\f\fR
form feed (0x0c)
.TP
\fB\\b\fR
backspace (0x08)
.TP
\fB\\a\fR
alarm, bell (0x07)
.TP
\fB\\e\fR
escape (0x1b)
.TP
\fB\\0\fR
binary zero (0x00)

.SS "Options"
.RB "The options " "gmisx" " are supported. " "e" " is not, since it would"
.RB "require a Perl interpreter and neither is " o ", because the pattern
is explicitly compiled, anyway. Additionally,
.SM PCRS
.RB "honors the options " "U" " and " "T" "."
Where
.SM PCRE
.RB "options are mentioned below, refer to " PCRE(3) " for the subtle differences"
to Perl behaviour.

.TP
.B g
.RB "Replace " all " instances of"
.IR pattern " in " subject ,
not just the first one.

.TP
.B i
.RI "Match the " pattern " without respect to case. This translates to"
.SM PCRE_CASELESS.

.TP
.B m
.RI "Treat the " subject " as consisting of multiple lines, i.e."
.RB ' ^ "' matches immediately after, and '" $ "' immediately before each newline."
Translates to
.SM PCRE_MULTILINE.

.TP
.B s
.RI "Treat the " subject " as consisting of one single line, i.e."
.RB "let the scope of the '" . "' metacharacter include newlines."
Translates to
.SM PCRE_DOTALL.

.TP
.B x
.RI "Allow extended regular expression syntax in the " pattern ","
.RB "enabling whitespace and comments in complex patterns."
Translates to
.SM PCRE_EXTENDED.

.TP
.B U
.RB "Switch the default behaviour of the '" * "' and '" + "' quantifiers"
.RB "to ungreedy. Note that appending a '" ? "' switches back to greedy(!)."
.RB "The explicit in-pattern switches " (?U) " and " (?-U) " remain unaffected."
Translates to
.SM PCRE_UNGREEDY.

.TP
.B T
.RI "Consider the " substitute " trivial, i.e. do not interpret any references"
or special character escape sequences in the substitute. Handy for large
user-supplied substitutes, which would otherwise have to be examined and properly
quoted.

.PP
Unsupported options are silently ignored.

.SH EXECUTING JOBS

.RI "Calling " pcrs_execute() " produces a modified copy of the " subject ", in which"
.RB "the first (or all, if the '" g "' option was given when compiling the job)"
.RI "occurance(s) of the job's " pattern " in the " subject " is replaced by the job's"
.IR substitute .

.RI "The first " subject_length " bytes following " subject " are processed, so"
.RI  "a " subject_length " that exceeds the actual " subject " is dangerous."
Note that if you want to get your zero-terminated C strings back including their
.RI "termination, you must let " subject_length " include the binary zero, i.e."
set it to
.BI strlen( subject ") + 1."

.RI "The " subject " itself is left untouched, and the " *result " is dynamically"
.RB "allocated, so it is the caller's responsibility to " free() " it when it's"
no longer needed.

.RI "The result's length is written to " *result_length "."

.RB "If the job matched, the " PCRS_SUCCESS " flag in"
.IB job ->flags
is set.

.SS Return value and diagnostics

.RB "On success, " pcrs_execute() " returns the number of substitutions that"
were made, which is limited to 0 or 1 for non-global searches.
.RI "On failure, a negative error code is returned and " *result " is set"
.RB "to " NULL .

.SH FREEING JOBS
.RB "It is not sufficient to call " free() " on a " pcrs_job ", because it "
contains pointers to other dynamically allocated structures.
.RB "Use " pcrs_free() " instead. It is safe to pass " NULL " pointers "
.RB "(or pointers to invalid " pcrs_job "s that contain " NULL " pointers"
.RB "to dependant structures) to " pcrs_free() "."

.SS Return value
.RB "The value of the job's " next " pointer."


.SH CHAINING JOBS

.SM PCRS
.RB "supports to some extent the chaining of multiple " pcrs_job " structures by"
.RB "means of their " next " member."

Chaining the jobs is up to you, but once you have built a linked list of jobs,
.RI "you can execute a whole " joblist " on a given subject by"
.RB "a single call to " pcrs_execute_list() ", which will sequentially traverse"
.RB "the linked list until it reaches a " NULL " pointer, and call " pcrs_execute() 
.RI "for each job it encounters, feeding the " result  " and " result_length " of each"
.RI "call into the next as the " subject " and " subject_length ". As in the single"
.RI "job case, the original " subject " remains untouched, but all interim " result "s"
.RB "are of course " free() "d. The return value is the accumulated number of matches"
.RI "for all jobs in the " joblist "."
.RI "Note that while this is handy, it reduces the diagnostic value of " err ", since "
you won't know which job failed.

.RI "In analogy, you can free all jobs in a given " joblist " by calling"
.BR pcrs_free_joblist() .

.SH QUOTING
The quote character is (surprise!) '\fB\\\fR'. It quotes the delimiter in a
.IR command ", the"
.RB ' $ "' in  a"
.IR substitute ", and, of course, itself. Note that the"
.RB ' $ "'doesn't need to be quoted if it isn't followed by " [0-9+'`&] "."

.RI "For quoting in the " pattern ", please refer to"
.BR PCRE(3) .

.SH DIAGNOSTICS

.RB "When " compiling " a job either via the " pcrs_compile() " or " pcrs_compile_command()
.RB "functions, you know that something went wrong when you are returned a " NULL " pointer."
.RI "In that case, or in the event of non-fatal warnings, the integer pointed to by " err
contains a nonzero error code, which is either a passed-through
.SM PCRE
error code or one generated by
.SM PCRS.
Under normal circumstances, it can take the following values:
.TP
.B PCRE_ERROR_NOMEMORY
While compiling the pattern,
.SM PCRE
ran out of memory.
.TP  
.B  PCRS_ERR_NOMEM
While compiling the job,
.SM PCRS
ran out of memory.
.TP  
.B  PCRS_ERR_CMDSYNTAX
.BR pcrs_compile_command() " didn't find four tokens while parsing the"
.IR command .
.TP  
.B  PCRS_ERR_STUDY
A
.SM PCRE
.RB "error occured while studying the compiled pattern. Since " pcre_study()
only provides textual diagnostic information, the details are lost.
.TP  
.B  PCRS_WARN_BADREF
.RI "The " substitute " contains a reference to a capturing subpattern that"
.RI "has a higher index than the number of capturing subpatterns in the " pattern
or that exceeds the current hard limit of 33 (See LIMITATIONS below). As in Perl,
this is non-fatal and results in substitutions with the empty string.

.PP
.RB "When " executing " jobs via " pcrs_execute() " or " pcrs_execute_list() ","
.RI "a negative return code indicates an error. In that case, *" result
.RB "is " NULL ". Possible error codes are:"
.TP
.B PCRE_ERROR_NOMEMORY
While matching the pattern,
.SM PCRE
ran out of memory. This can only happen if there are more than 33 backrefrences
.RI "in the " pattern "(!)"
.BR and " memory is too tight to extend storage for more."
.TP  
.B  PCRS_ERR_NOMEM
While executing the job,
.SM PCRS
ran out of memory.
.TP  
.B  PCRS_ERR_BADJOB
.RB "The " pcrs_job "*  passed to " pcrs_execute " was NULL, or the"
.RB "job is bogus (it contains " NULL " pointers to the compiled
pattern, extra, or substitute).

.PP
If you see any other
.SM PCRE
error code passed through, you've either messed with the compiled job
or found a bug in
.SM PCRS.
Please send me an email.

.RB "Ah, and don't look for " PCRE_ERROR_NOMATCH ", since this"
is not an error in the context of
.SM PCRS.
.RI "Should there be no match, an exact copy of the " subject " is"
.RI "found at *" result " and the return code is 0 (matches)."

All error codes can be translated into human readable text by means
.RB "of the " pcrs_strerror() " function."


.SH EXAMPLE
A trivial command-line test program for
.SM PCRS
might look like:

.nf
#include <pcrs.h>
#include <stdio.h>

int main(int Argc, char **Argv)
{
   pcrs_job *job;
   char *result;
   int newsize, err;

   if (Argc != 3)
   {
      fprintf(stderr, "Usage: %s s/pattern/substitute/[options]  subject\\n", Argv[0]);
      return 1;
   }

   if (NULL == (job = pcrs_compile_command(Argv[1], &err)))
   {
      printf("Compile error:  %s (%d).\\n", pcrs_strerror(err), err);
   }

   if (0 > (err = pcrs_execute(job, Argv[2], strlen(Argv[2]) + 1, &result, &newsize)))
   {
      printf("Exec error:  %s (%d).\\n", pcrs_strerror(err), err);
   }

   /* Will tolerate NULL result */
   printf("Result: *%s*\\n", result);

   pcrs_free_job(job);
   if (result) free(result);

   return 0;
}

.fi


.SH LIMITATIONS
The number of matches that a global job can have is only limited by the
available memory. An initial storage for 40 matches is reserved, which
is dynamically resized by the factor 1.6 if exhausted.

The number of capturing subpatterns is currently limited to 33, which
is a Bad Thing[tm]. It should be dynamically expanded until it reaches the
.SM PCRE
limit of 99.

All of the above values can be adjusted in the "Capacity" section
.RB "of " pcrs.h "."

The Perl-style escape sequences for special characters \\\fInnn\fR,
\\x\fInn\fR, and \\c\fIX\fR are currently unsupported.

.SH BUGS
This library has only been tested in the context of one application
and should be considered high risk.

.SH HISTORY
.SM PCRS
was originally written for the Internet Junkbuster project
(http://sourceforge.net/projects/ijbswa/).

.SH SEE ALSO
.B PCRE(3), perl(1), perlre(1)

.SH AUTHOR

.SM PCRS
is Copyright 2000, 2001 by Andreas Oesterhelt <oes@oesterhelt.org> and is
licensed under the Terms of the GNU Lesser General Public License (LGPL), which
should be included in this distribution.

If not, refer to http://www.gnu.org/licenses/lgpl.html or write to the Free
Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.