kenmirrors/freeswitch
1
0
Fork 0
mirror of https://github.com/signalwire/freeswitch.git synced 2025-08-13 01:26:58 +00:00
Code Issues Projects Releases Wiki Activity

update to pcre 7.9

Browse Source 
git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@13706 d0543943-73ff-0310-b7d9-9358b9ac24b2
This commit is contained in:
Michael Jerris
2009-06-08 23:51:30 +00:00
parent a1e5add731
commit f7efdaa901
178 changed files with 43560 additions and 11382 deletions
Download Patch File Download Diff File Expand all files Collapse all files

100
libs/pcre/doc/html/pcrematching.html
View File

@@ -16,9 +16,11 @@ man page, in case the conversion went wrong.
<li><a name="TOC1" href="#SEC1">PCRE MATCHING ALGORITHMS</a>
<li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a>
<li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a>
<li><a name="TOC4" href="#SEC4">THE DFA MATCHING ALGORITHM</a>
<li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE DFA ALGORITHM</a>
<li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE DFA ALGORITHM</a>
<li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a>
<li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
<li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
<li><a name="TOC7" href="#SEC7">AUTHOR</a>
<li><a name="TOC8" href="#SEC8">REVISION</a>
</ul>
<br><a name="SEC1" href="#TOC1">PCRE MATCHING ALGORITHMS</a><br>
<P>
@@ -46,7 +48,7 @@ is matched against the string
&#60;something&#62; &#60;something else&#62; &#60;something further&#62;
</pre>
there are three possible answers. The standard algorithm finds only one of
them, whereas the DFA algorithm finds all three.
them, whereas the alternative algorithm finds all three.
</P>
<br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
<P>
@@ -59,8 +61,8 @@ correspond to the two matching algorithms provided by PCRE.
</P>
<br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
<P>
In the terminology of Jeffrey Friedl's book \fIMastering Regular
Expressions\fP, the standard algorithm is an "NFA algorithm". It conducts a
In the terminology of Jeffrey Friedl's book "Mastering Regular
Expressions", the standard algorithm is an "NFA algorithm". It conducts a
depth-first search of the pattern tree. That is, it proceeds along a single
path through the tree, checking that the subject matches what is required. When
there is a mismatch, the algorithm tries any alternatives at the current point,
@@ -83,14 +85,15 @@ straightforward for this algorithm to keep track of the substrings that are
matched by portions of the pattern in parentheses. This provides support for
capturing parentheses and back references.
</P>
<br><a name="SEC4" href="#TOC1">THE DFA MATCHING ALGORITHM</a><br>
<br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
<P>
DFA stands for "deterministic finite automaton", but you do not need to
understand the origins of that name. This algorithm conducts a breadth-first
search of the tree. Starting from the first matching point in the subject, it
scans the subject string from left to right, once, character by character, and
as it does this, it remembers all the paths through the tree that represent
valid matches.
This algorithm conducts a breadth-first search of the tree. Starting from the
first matching point in the subject, it scans the subject string from left to
right, once, character by character, and as it does this, it remembers all the
paths through the tree that represent valid matches. In Friedl's terminology,
this is a kind of "DFA algorithm", though it is not implemented as a
traditional finite state machine (it keeps multiple states active
simultaneously).
</P>
<P>
The scan continues until either the end of the subject is reached, or there are
@@ -114,12 +117,21 @@ matches that start at later positions.
</P>
<P>
There are a number of features of PCRE regular expressions that are not
supported by the DFA matching algorithm. They are as follows:
supported by the alternative matching algorithm. They are as follows:
</P>
<P>
1. Because the algorithm finds all possible matches, the greedy or ungreedy
nature of repetition quantifiers is not relevant. Greedy and ungreedy
quantifiers are treated in exactly the same way.
quantifiers are treated in exactly the same way. However, possessive
quantifiers can make a difference when what follows could also match what is
quantified, for example in a pattern like this:
<pre>
^a++\w!
</pre>
This pattern matches "aaab!" but not "aaa!", which would be matched by a
non-possessive quantifier. Similarly, if an atomic group is present, it is
matched as if it were a standalone pattern at the current point, and the
longest match is then "locked in" for the rest of the overall pattern.
</P>
<P>
2. When dealing with multiple paths through the tree simultaneously, it is not
@@ -133,22 +145,30 @@ not supported, and cause errors if encountered.
</P>
<P>
4. For the same reason, conditional expressions that use a backreference as the
condition are not supported.
condition or test for a specific group recursion are not supported.
</P>
<P>
5. Callouts are supported, but the value of the <i>capture_top</i> field is
5. Because many paths through the tree may be active, the \K escape sequence,
which resets the start of the match when encountered (but may be on some paths
and not on others), is not supported. It causes an error if encountered.
</P>
<P>
6. Callouts are supported, but the value of the <i>capture_top</i> field is
always 1, and the value of the <i>capture_last</i> field is always -1.
</P>
<P>
6.
The \C escape sequence, which (in the standard algorithm) matches a single
byte, even in UTF-8 mode, is not supported because the DFA algorithm moves
through the subject string one character at a time, for all active paths
7. The \C escape sequence, which (in the standard algorithm) matches a single
byte, even in UTF-8 mode, is not supported because the alternative algorithm
moves through the subject string one character at a time, for all active paths
through the tree.
</P>
<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE DFA ALGORITHM</a><br>
<P>
Using the DFA matching algorithm provides the following advantages:
8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
supported. (*FAIL) is supported, and behaves like a failing negative assertion.
</P>
<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
<P>
Using the alternative matching algorithm provides the following advantages:
</P>
<P>
1. All possible matches (at a single point in the subject) are automatically
@@ -159,17 +179,18 @@ callouts.
<P>
2. There is much better support for partial matching. The restrictions on the
content of the pattern that apply when using the standard algorithm for partial
matching do not apply to the DFA algorithm. For non-anchored patterns, the
starting position of a partial match is available.
matching do not apply to the alternative algorithm. For non-anchored patterns,
the starting position of a partial match is available.
</P>
<P>
3. Because the DFA algorithm scans the subject string just once, and never
needs to backtrack, it is possible to pass very long subject strings to the
matching function in several pieces, checking for partial matching each time.
3. Because the alternative algorithm scans the subject string just once, and
never needs to backtrack, it is possible to pass very long subject strings to
the matching function in several pieces, checking for partial matching each
time.
</P>
<br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE DFA ALGORITHM</a><br>
<br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
<P>
The DFA algorithm suffers from a number of disadvantages:
The alternative algorithm suffers from a number of disadvantages:
</P>
<P>
1. It is substantially slower than the standard algorithm. This is partly
@@ -180,13 +201,24 @@ less susceptible to optimization.
2. Capturing parentheses and back references are not supported.
</P>
<P>
3. The "atomic group" feature of PCRE regular expressions is supported, but
does not provide the advantage that it does for the standard algorithm.
3. Although atomic groups are supported, their use does not provide the
performance advantage that it does for the standard algorithm.
</P>
<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
<P>
Last updated: 06 June 2006
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><a name="SEC8" href="#TOC1">REVISION</a><br>
<P>
Last updated: 19 April 2008
<br>
Copyright &copy; 1997-2008 University of Cambridge.
<br>
Copyright &copy; 1997-2006 University of Cambridge.
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>