Removed the documentation files because they were making the httpd

The documentation directory has been omitted from this copy of PCRE

inside the httpd codebase because it's huge--over a megabyte of PCRE docs.

The PCRE documentation directory is available in unmodified form in the

vendor branch.  You can access it via web browser or Subversion checkout at

http://svn.apache.org/repos/asf/httpd/httpd/vendor/pcre/current/doc/

Technical Notes about PCRE

--------------------------

Historical note 1

-----------------

Many years ago I implemented some regular expression functions to an algorithm

suggested by Martin Richards. These were not Unix-like in form, and were quite

restricted in what they could do by comparison with Perl. The interesting part

about the algorithm was that the amount of space required to hold the compiled

form of an expression was known in advance. The code to apply an expression did

not operate by backtracking, as the original Henry Spencer code and current

Perl code does, but instead checked all possibilities simultaneously by keeping

a list of current states and checking all of them as it advanced through the

subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA

algorithm". When the pattern was all used up, all remaining states were

possible matches, and the one matching the longest subset of the subject string

was chosen. This did not necessarily maximize the individual wild portions of

the pattern, as is expected in Unix and Perl-style regular expressions.

Historical note 2

-----------------

By contrast, the code originally written by Henry Spencer and subsequently

heavily modified for Perl actually compiles the expression twice: once in a

dummy mode in order to find out how much store will be needed, and then for

real. The execution function operates by backtracking and maximizing (or,

optionally, minimizing in Perl) the amount of the subject that matches

individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's

terminology.

OK, here's the real stuff

-------------------------

For the set of functions that forms PCRE (which are unrelated to those

mentioned above), I tried at first to invent an algorithm that used an amount

of store bounded by a multiple of the number of characters in the pattern, to

save on compiling time. However, because of the greater complexity in Perl

regular expressions, I couldn't do this. In any case, a first pass through the

pattern is needed, for a number of reasons. PCRE works by running a very

degenerate first pass to calculate a maximum store size, and then a second pass

to do the real compile - which may use a bit less than the predicted amount of

store. The idea is that this is going to turn out faster because the first pass

is degenerate and the second pass can just store stuff straight into the

vector. It does make the compiling functions bigger, of course, but they have

got quite big anyway to handle all the Perl stuff.

The compiled form of a pattern is a vector of bytes, containing items of

variable length. The first byte in an item is an opcode, and the length of the

item is either implicit in the opcode or contained in the data bytes that

follow it. 

In many cases below "two-byte" data values are specified. This is in fact just

a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the

performance). This is necessary only when patterns whose compiled length is

greater than 64K are going to be processed. In this description, we assume the 

"normal" compilation options.

A list of all the opcodes follows:

Opcodes with no following data

------------------------------

These items are all just one byte long

  OP_END                 end of pattern

  OP_ANY                 match any character

  OP_ANYBYTE             match any single byte, even in UTF-8 mode

  OP_SOD                 match start of data: \A

  OP_SOM,                start of match (subject + offset): \G

  OP_CIRC                ^ (start of data, or after \n in multiline)

  OP_NOT_WORD_BOUNDARY   \W

  OP_WORD_BOUNDARY       \w

  OP_NOT_DIGIT           \D

  OP_DIGIT               \d

  OP_NOT_WHITESPACE      \S

  OP_WHITESPACE          \s

  OP_NOT_WORDCHAR        \W

  OP_WORDCHAR            \w

  OP_EODN                match end of data or \n at end: \Z

  OP_EOD                 match end of data: \z

  OP_DOLL                $ (end of data, or before \n in multiline)

  OP_EXTUNI              match an extended Unicode character 

Repeating single characters

---------------------------

The common repeats (*, +, ?) when applied to a single character use the

following opcodes:

  OP_STAR

  OP_MINSTAR

  OP_PLUS

  OP_MINPLUS

  OP_QUERY

  OP_MINQUERY

In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.

Those with "MIN" in their name are the minimizing versions. Each is followed by

the character that is to be repeated. Other repeats make use of

  OP_UPTO

  OP_MINUPTO

  OP_EXACT

which are followed by a two-byte count (most significant first) and the

repeated character. OP_UPTO matches from 0 to the given number. A repeat with a

non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an

OP_UPTO (or OP_MINUPTO).

Repeating character types

-------------------------

Repeats of things like \d are done exactly as for single characters, except

that instead of a character, the opcode for the type is stored in the data

byte. The opcodes are:

  OP_TYPESTAR

  OP_TYPEMINSTAR

  OP_TYPEPLUS

  OP_TYPEMINPLUS

  OP_TYPEQUERY

  OP_TYPEMINQUERY

  OP_TYPEUPTO

  OP_TYPEMINUPTO

  OP_TYPEEXACT

Match by Unicode property

-------------------------

OP_PROP and OP_NOTPROP are used for positive and negative matches of a 

character by testing its Unicode property (the \p and \P escape sequences).

Each is followed by a single byte that encodes the desired property value.

Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two 

bytes: OP_PROP or OP_NOTPROP and then the desired property value.

Matching literal characters

---------------------------

The OP_CHAR opcode is followed by a single character that is to be matched 

casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the 

character may be more than one byte long. (Earlier versions of PCRE used 

multi-character strings, but this was changed to allow some new features to be 

added.)

Character classes

-----------------

If there is only one character, OP_CHAR or OP_CHARNC is used for a positive

class, and OP_NOT for a negative one (that is, for something like [^a]).

However, in UTF-8 mode, the use of OP_NOT applies only to characters with

values < 128, because OP_NOT is confined to single bytes.

Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,

negated, single-character class. The normal ones (OP_STAR etc.) are used for a

repeated positive single-character class.

When there's more than one character in a class and all the characters are less

than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative

one. In either case, the opcode is followed by a 32-byte bit map containing a 1

bit for every character that is acceptable. The bits are counted from the least

significant end of each byte.

The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,

subject characters with values greater than 256 can be handled correctly. For

OP_CLASS they don't match, whereas for OP_NCLASS they do.

For classes containing characters with values > 255, OP_XCLASS is used. It

optionally uses a bit map (if any characters lie within it), followed by a list

of pairs and single characters. There is a flag character than indicates

whether it's a positive or a negative class.

Back references

---------------

OP_REF is followed by two bytes containing the reference number.

Repeating character classes and back references

-----------------------------------------------

Single-character classes are handled specially (see above). This applies to

OP_CLASS and OP_REF. In both cases, the repeat information follows the base

item. The matching code looks at the following opcode to see if it is one of

  OP_CRSTAR

  OP_CRMINSTAR

  OP_CRPLUS

  OP_CRMINPLUS

  OP_CRQUERY

  OP_CRMINQUERY

  OP_CRRANGE

  OP_CRMINRANGE

All but the last two are just single-byte items. The others are followed by

four bytes of data, comprising the minimum and maximum repeat counts.

Brackets and alternation

------------------------

A pair of non-capturing (round) brackets is wrapped round each expression at

compile time, so alternation always happens in the context of brackets.

Non-capturing brackets use the opcode OP_BRA, while capturing brackets use

OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English

speakers, including myself, can be round, square, curly, or pointy. Hence this

usage.]

Originally PCRE was limited to 99 capturing brackets (so as not to use up all

the opcodes). From release 3.5, there is no limit. What happens is that the

first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as

above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the

first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket

number. This opcode is ignored while matching, but is fished out when handling

the bracket itself. (They could have all been done like this, but I was making

minimal changes.)

A bracket opcode is followed by two bytes which give the offset to the next

alternative OP_ALT or, if there aren't any branches, to the matching OP_KET

opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,

or to the OP_KET opcode.

OP_KET is used for subpatterns that do not repeat indefinitely, while

OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or

maximally respectively. All three are followed by two bytes giving (as a

positive number) the offset back to the matching OP_BRA opcode.

If a subpattern is quantified such that it is permitted to match zero times, it

is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte

opcodes which tell the matcher that skipping this subpattern entirely is a

valid branch.

A subpattern with an indefinite maximum repetition is replicated in the

compiled data its minimum number of times (or once with OP_BRAZERO if the

minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX

as appropriate.

A subpattern with a bounded maximum repetition is replicated in a nested

fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO

before each replication after the minimum, so that, for example, (abc){2,5} is

compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.

Assertions

----------

Forward assertions are just like other subpatterns, but starting with one of

the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes

OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion

is OP_REVERSE, followed by a two byte count of the number of characters to move

back the pointer in the subject string. When operating in UTF-8 mode, the count

is a character count rather than a byte count. A separate count is present in

each alternative of a lookbehind assertion, allowing them to have different

fixed lengths.

Once-only subpatterns

---------------------

These are also just like other subpatterns, but they start with the opcode

OP_ONCE.

Conditional subpatterns

-----------------------

These are like other subpatterns, but they start with the opcode OP_COND. If

the condition is a back reference, this is stored at the start of the

subpattern using the opcode OP_CREF followed by two bytes containing the

reference number. If the condition is "in recursion" (coded as "(?(R)"), the

same scheme is used, with a "reference number" of 0xffff. Otherwise, a

conditional subpattern always starts with one of the assertions.

Recursion

---------

Recursion either matches the current regex, or some subexpression. The opcode

OP_RECURSE is followed by an value which is the offset to the starting bracket

from the start of the whole pattern.

Callout

-------

OP_CALLOUT is followed by one byte of data that holds a callout number in the

range 0 to 254 for manual callouts, or 255 for an automatic callout. In both 

cases there follows a two-byte value giving the offset in the pattern to the

start of the following item, and another two-byte item giving the length of the

next item.

Changing options

----------------

If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT

opcode is compiled, followed by one byte containing the new settings of these

flags. If there are several alternatives, there is an occurrence of OP_OPT at

the start of all those following the first options change, to set appropriate

options for the start of the alternative. Immediately after the end of the

group there is another such item to reset the flags to their previous values. A

change of flag right at the very start of the pattern can be handled entirely

at compile time, and so does not cause anything to be put into the compiled

data.

Philip Hazel

September 2004

<html>

<head>

<title>PCRE specification</title>

</head>

<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">

<h1>Perl-compatible Regular Expressions (PCRE)</h1>

<p>

The HTML documentation for PCRE comprises the following pages:

</p>

<table>

<tr><td><a href="pcre.html">pcre</a></td>

    <td>&nbsp;&nbsp;Introductory page</td></tr>

<tr><td><a href="pcreapi.html">pcreapi</a></td>

    <td>&nbsp;&nbsp;PCRE's native API</td></tr>

<tr><td><a href="pcrebuild.html">pcrebuild</a></td>

    <td>&nbsp;&nbsp;Options for building PCRE</td></tr>

<tr><td><a href="pcrecallout.html">pcrecallout</a></td>

    <td>&nbsp;&nbsp;The <i>callout</i> facility</td></tr>

<tr><td><a href="pcrecompat.html">pcrecompat</a></td>

    <td>&nbsp;&nbsp;Compability with Perl</td></tr>

<tr><td><a href="pcregrep.html">pcregrep</a></td>

    <td>&nbsp;&nbsp;The <b>pcregrep</b> command</td></tr>

<tr><td><a href="pcrepartial.html">pcrepartial</a></td>

    <td>&nbsp;&nbsp;Using PCRE for partial matching</td></tr>

<tr><td><a href="pcrepattern.html">pcrepattern</a></td>

    <td>&nbsp;&nbsp;Specification of the regular expressions supported by PCRE</td></tr>

<tr><td><a href="pcreperform.html">pcreperform</a></td>

    <td>&nbsp;&nbsp;Some comments on performance</td></tr>

<tr><td><a href="pcreposix.html">pcreposix</a></td>

    <td>&nbsp;&nbsp;The POSIX API to the PCRE library</td></tr>

<tr><td><a href="pcreprecompile.html">pcreprecompile</a></td>

    <td>&nbsp;&nbsp;How to save and re-use compiled patterns</td></tr>

<tr><td><a href="pcresample.html">pcresample</a></td>

    <td>&nbsp;&nbsp;Description of the sample program</td></tr>

<tr><td><a href="pcretest.html">pcretest</a></td>

    <td>&nbsp;&nbsp;The <b>pcretest</b> command for testing PCRE</td></tr>

</table>

<p>

There are also individual pages that summarize the interface for each function

in the library:

</p>

<table>

<tr><td><a href="pcre_compile.html">pcre_compile</a></td>

    <td>&nbsp;&nbsp;Compile a regular expression</td></tr>

<tr><td><a href="pcre_config.html">pcre_config</a></td>

    <td>&nbsp;&nbsp;Show build-time configuration options</td></tr>

<tr><td><a href="pcre_copy_named_substring.html">pcre_copy_named_substring</a></td>

    <td>&nbsp;&nbsp;Extract named substring into given buffer</td></tr>

<tr><td><a href="pcre_copy_substring.html">pcre_copy_substring</a></td>

    <td>&nbsp;&nbsp;Extract numbered substring into given buffer</td></tr>

<tr><td><a href="pcre_exec.html">pcre_exec</a></td>

    <td>&nbsp;&nbsp;Match a compiled pattern to a subject string</td></tr>

<tr><td><a href="pcre_free_substring.html">pcre_free_substring</a></td>

    <td>&nbsp;&nbsp;Free extracted substring</td></tr>

<tr><td><a href="pcre_free_substring_list.html">pcre_free_substring_list</a></td>

    <td>&nbsp;&nbsp;Free list of extracted substrings</td></tr>

<tr><td><a href="pcre_fullinfo.html">pcre_fullinfo</a></td>

    <td>&nbsp;&nbsp;Extract information about a pattern</td></tr>

<tr><td><a href="pcre_get_named_substring.html">pcre_get_named_substring</a></td>

    <td>&nbsp;&nbsp;Extract named substring into new memory</td></tr>

<tr><td><a href="pcre_get_stringnumber.html">pcre_get_stringnumber</a></td>

    <td>&nbsp;&nbsp;Convert captured string name to number</td></tr>

<tr><td><a href="pcre_get_substring.html">pcre_get_substring</a></td>

    <td>&nbsp;&nbsp;Extract numbered substring into new memory</td></tr>

<tr><td><a href="pcre_get_substring_list.html">pcre_get_substring_list</a></td>

    <td>&nbsp;&nbsp;Extract all substrings into new memory</td></tr>

<tr><td><a href="pcre_info.html">pcre_info</a></td>

    <td>&nbsp;&nbsp;Obsolete information extraction function</td></tr>

<tr><td><a href="pcre_maketables.html">pcre_maketables</a></td>

    <td>&nbsp;&nbsp;Build character tables in current locale</td></tr>

<tr><td><a href="pcre_study.html">pcre_study</a></td>

    <td>&nbsp;&nbsp;Study a compiled pattern</td></tr>

<tr><td><a href="pcre_version.html">pcre_version</a></td>

    <td>&nbsp;&nbsp;Return PCRE version and release date</td></tr>

</table>

</html>

<html>

<head>

<title>pcre_compile specification</title>

</head>

<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">

<h1>pcre_compile man page</h1>

<p>

Return to the <a href="index.html">PCRE index page</a>.

</p>

<p>

This page is part of the PCRE HTML documentation. It was generated automatically

from the original man page. If there is any nonsense in it, please consult the

man page, in case the conversion went wrong.

<br>

<br><b>

SYNOPSIS

</b><br>

<P>

<b>#include &#60;pcre.h&#62;</b>

</P>

<P>

<b>pcre *pcre_compile(const char *<i>pattern</i>, int <i>options</i>,</b>

<b>const char **<i>errptr</i>, int *<i>erroffset</i>,</b>

<b>const unsigned char *<i>tableptr</i>);</b>

</P>

<br><b>

DESCRIPTION

</b><br>

<P>

This function compiles a regular expression into an internal form. Its

arguments are:

<pre>

  <i>pattern</i>       A zero-terminated string containing the

                  regular expression to be compiled

  <i>options</i>       Zero or more option bits

  <i>errptr</i>        Where to put an error message

  <i>erroffset</i>     Offset in pattern where error was found

  <i>tableptr</i>      Pointer to character tables, or NULL to

                  use the built-in default

</pre>

The option bits are:

<pre>

  PCRE_ANCHORED         Force pattern anchoring

  PCRE_AUTO_CALLOUT     Compile automatic callouts

  PCRE_CASELESS         Do caseless matching

  PCRE_DOLLAR_ENDONLY   $ not to match newline at end

  PCRE_DOTALL           . matches anything including NL

  PCRE_EXTENDED         Ignore whitespace and # comments

  PCRE_EXTRA            PCRE extra features

                          (not much use currently)

  PCRE_MULTILINE        ^ and $ match newlines within data

  PCRE_NO_AUTO_CAPTURE  Disable numbered capturing paren-

                          theses (named ones available)

  PCRE_UNGREEDY         Invert greediness of quantifiers

  PCRE_UTF8             Run in UTF-8 mode

  PCRE_NO_UTF8_CHECK    Do not check the pattern for UTF-8

                          validity (only relevant if

                          PCRE_UTF8 is set)

</pre>

PCRE must be built with UTF-8 support in order to use PCRE_UTF8 and

PCRE_NO_UTF8_CHECK.

</P>

<P>

The yield of the function is a pointer to a private data structure that

contains the compiled pattern, or NULL if an error was detected.

</P>

<P>

There is a complete description of the PCRE native API in the

<a href="pcreapi.html"><b>pcreapi</b></a>

page and a description of the POSIX API in the

<a href="pcreposix.html"><b>pcreposix</b></a>

page.

<p>

Return to the <a href="index.html">PCRE index page</a>.

</p>