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<title>Design of the regex processing framework</title>
<h1 align=center><a name="Top"></a>Regex Processing Framework</h1>
<p class="TOCHeading"><A href="#Revision_History">Revision History</A></p>
<p class="TOCHeading"><A href="#Disclaimer">Disclaimer</A></p>
<p class="TOCHeading"><A href="#About_this_document">About this Document</A>
<p class="TOC"><A href="#Purpose">Purpose </A> </p>
<p class="TOC"><A href="#Intended_Audience">Intended Audience </A> </p>
<p class="TOC"><A href="#Documentation_Conventions">Documentation Conventions</A></p>
<p class="TOCHeading"><a href="#Introduction">Introduction to Pattern Matching</a></p>
<p class="TOCHeading"><a href="#AbstractSetIntro">AbstractSet Interface</a></p>
<p class="TOC"><a href="#Characteristics">Characteristics</a></p>
<p class="TOC"><a href="#MethodsUsed">Methods Used</a></p>
<p class="TOC"><a href="#ClassHierarchy">Class Hierarchy</a></p>
<p class="TOC"><a href="#Optimizations">Optimizations</a></p>
<p class="TOCHeading"><a href="#UsageExamples">Usage Examples</a></p>
<p class="TOCHeading"><A href="#References">References</A> </p>
<H1><A name="Revision_History"></A>Revision History</H1>
<TABLE width="100%">
<TD class="TableHeading" width="25%"> Version </TD>
<TD class="TableHeading" width="50%"> Version Information </TD>
<TD class="TableHeading"> Date </TD>
<TD class="TableCell" width="25%"> Initial version </TD>
<TD class="TableCell" width="25%"> Nadya Morozova, Nikolay Kuznetsov: document
<TD class="TableCell"> December 16, 2005</TD>
<H1><A name="Disclaimer"></A>Disclaimer and Legal Information</H1>
<P>Copyright 2005 The Apache Software Foundation or its licensors, as applicable.</P>
<P>Licensed under the Apache License, Version 2.0 (the &quot;License&quot;); you
may not use this file except in compliance with the License. You may obtain
a copy of the License at <A href=""></A>. </P>
<P>Unless required by applicable law or agreed to in writing, software distributed
under the License is distributed on an &quot;AS IS&quot; BASIS, WITHOUT WARRANTIES
OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
specific language governing permissions and limitations under the License.</P>
<h1><a name="About_this_document"></a>About This Document</h1>
<h2><a name="Purpose"></a>Purpose</h2>
<p>This document describes the <code>java.util.regex</code> package delivered as part of
the Harmony classlibrary project. This document provides an overview
of the overall architecture with focus on the aspects improving performance.
<h2><a name="Intended_Audience"></a>Intended Audience</h2>
<p>The target audience for the document includes a wide community of engineers
interested in using regular expression package and in further work with the
product to contribute to its development. The document assumes that readers
are familiar with Regular expressions [<a href="#Fri02">1</a>, <a href="#MY60">2</a>,
<a href="#THO68">3</a>], finite automata theory [<a href="#ASU86">4</a>], basic
compiler techniques [<a href="#ASU86">4</a>] and the Java<a
href="#*">*</a> programming language [<a href="#JavaSite">5</a>]. </p>
<h2><a name="Documentation_Conventions"></a>Documentation Conventions</h2>
<p>This document uses the <a
href="../conventions.htm">unified conventions</a> for the Harmony documentation kit.</p>
<P class="backtotop"><A href="#Top">Back to Top</A></P>
<h1><a name="Introduction"></a>Introduction to Pattern Matching </h1>
<p>To analyze text in search of sequences matching preset patterns, you can choose
from a variety of techniques, such as the wide-spread <i>regular expressions</i>
(RE) [<a href="#Fri02">1</a>], <em>exact string matching</em>, for example, the Boyer-Moore algorithm
(BM) [<a href="#BM77">6</a>], and others. However, the RE engine is rather complex, and significantly
impacts performance, whereas the exact string matching techniques have a limited
application. </p>
<p>For example, the regular expression <code>.*world</code> is used to find the
last instance of <i>world</i> in a sentence. The BM string searching algorithm is more
appropriate for solving the task than the RE technique, and is more effective
from the performance perspective. To optimize using pattern matching techniques
with different tasks, Harmony provides a unified interface that applies to any part
of a regular expression, including the whole expression. </p>
<p>In terms of the Finite Automata theory, which is the basis of regular expression
processing, a part of regular expression is a <i>node</i>. The Harmony regex framework
treats every distinctive part of a regular expression and the whole expression
as a node. Each node implements the unified interface adjusted for a specific
technique. For instance, for the regular expression in the example, the Harmony
framework includes a special class SequenceSet, which has a unified interface
called <code>AbstractSet</code>, and implements the Boyer-Moore algorithm in
searching for a word.</p>
<P class="backtotop"><A href="#Top">Back to Top</A></P>
<h1><a name="AbstractSetIntro"></a>AbstractSet Interface</h1>
<p>The key feature of the Harmony regex framework the single super interface, <code>AbstractSet</code>,
shared by all types of nodes. Individual nodes are independent, so that every
node of the automaton incorporates its own find-match algorithm optimized for
a specific type of nodes. You can use methods implemented in the <code>AbstractSet</code>
interface subclasses or override these methods to use your own more efficient
<h2><a name="Characteristics"></a>Characteristics</h2>
<p>The <code>AbstractSet</code> interface has the following key characteristics:
<li> <b>Unified structure:</b> General contract for all nodes including node
chains. This enables the framework to support new features and optimizations
for a specific construction of RE without impacting other parts of the framework.
<li><b>Simplicity:</b> Every node of an automaton is simple and handles a specific
optimization or functionality. The resulting automaton is straight-forward
and includes no multi-purpose functionality. To enable support of new functionality,
create a new class.
<p class="note">NOTE: </p>
<p class="notetext">Because new features and optimizations are implemented
independently, expansion of the framework has no negative impact on the
overall performance. </p>
<li><b>Independence</b>:The matching and finding procedures are incorporated
into the interface and require no external logic. The interface allows using
the same find-match methods for all types of nodes, and at the same time enables
optimizing the find-match algorithm for individual node types to improve performance.
<P class="backtotop"><A href="#Top">Back to Top</A></P>
<h2><a name="MethodsUsed"></a>Methods Used </h2>
<p>The <code>AbstractSet</code> interface defines the matching and finding methods,
and the service methods supporting them, as follows. </p>
<p class="class">Service Methods</p>
<dd>Checks whether the current node matches the current string symbol(s) to
ensure that the automaton can proceed with the current state. The method returns
the number of accepted symbols. This method is designed for nodes representing
tokens, which mostly use the default match procedure. To optimize the match
procedure for a specific type of token, you only need to override the <code>accept()</code>
method, see <a href="#Example1">Example 1</a>.</dd>
<dd>Checks whether the first symbol of the current node intersects with previous
one. If not, then the previous node is quantified possessively without <i>backtrack</i>
(the option of restarting the search from the previous position). </dd>
<dd>Returns the next node of the automaton. </dd>
<p class="class">Matching and Finding Methods</p>
<dd>Runs the match procedure starting from the current state. This is the only
method you need to override in order to introduce new functionality. By default,
the <code>matches()</code> method does the following when working with terms (see <a href="#ClassHierarchy">Class
<li> Calls the <code>accept()</code> method and proceeds if the method returns
a positive value. </li>
<li> Calls the match of the next node with the shift obtained from the <code>accept()</code>
<li> Returns <code>TRUE</code> in case of a successful match. </li>
<dd>Checks whether the pattern can match any part of the string in the following
<li>Finds the next position in the input string, for which the <code>accept()</code>
method returns a positive value. If no matches are found, the method terminates
and returns a negative value.</li>
<li>From this position, runs the <code>matches()</code> method.</li>
<dd>Does the same as the <code>find()</code> method but<code></code> starts
from the end of the string or from the nearest new-line symbol. This method
optimizes the find procedure when the current node of the pattern fits the
rest of the string. In such cases, it is necessary to find the last occurrence
of the pattern represented by the next node, as in the case of the <code>.*world
</code> pattern. </dd>
<P class="backtotop"><A href="#Top">Back to Top</A></P>
<h2><a name="ClassHierarchy"></a>Class Hierarchy</h2>
<p><a href="#figure1">Figure 1</a> shows the class hierarchy based on the <code>AbstractSet</code>
interface. As mentioned in its <a href="#AbstractSetIntro">description</a>,
the whole hierarchy is based on this interface. The other classes of the framework
are divided into the following categories: </p>
<li><b>Tokens or Leafs</b> (<code>LeafSet</code> in the figure): nodes representing
ordinal symbols, substrings, ranges, character classes, and other basic units
of regular expressions.
<li><b>Quantifiers</b> (<code>QuantifierSet</code> in the figure): set of nodes responsible
for quantification of terms. Terms are simple and usually represent only one
symbol. Therefore, terms are quantified without recursion, and backtracks are
processed on the basis of the underlying pattern length.
<li><b>Groups </b>(<code>JointSet</code> in the figure): sets derived from parenthetic constructions.
These nodes represent alternations of other sets.
<li><b>Group Quantifiers </b>(<code>GroupQuantifier</code> in the figure): special
quantifiers over Groups. Because the length of a groups can vary, backtracks
require recursion.
<p align=center><img border=0 alt="Hierarchy of classes in regexp framework" src="images/picture1.gif"></p>
<p class="special"><a name="figure1"></a>Figure 1. Class Hierarchy</p>
<p>Figure 1 displays only the basics of the regex framework. This framework also
includes other nodes responsible for different optimizations. For more details,
see the comments inlined in code. </p>
<P class="backtotop"><A href="#Top">Back to Top</A></P>
<h2><a name="Optimizations"></a>Optimizations</h2>
<p>In the current implementation, most optimizations are based on the node representation
to improve efficiency of the framework. It is noteworthy that an optimal finite
automaton is built during compile time, so that the constructed automaton does
not spend additional time on decision-making overhead during match time. The
regex framework optimizations improve different aspects, such as: </p>
<li> <b>Character or String find methods: </b>The methods <code>find()</code>
and <code>findBack()</code> of certain nodes use exact pattern matching algorithms,
specifically, the Boyer-Moore fast string search algorithm.
<li> <b>Quantified dot term:</b> This term (<code>.*</code>) covers the rest
of the string and can therefore run <code>findBack()</code> from the end of
the string. If the <code>findBack()</code> method of the following node implements
a special algorithm, the pattern matching speed for this chain increases,
otherwise the operation runs at the same speed.
<li> <b>Quantified non-intersecting states:</b> If the quantified node does
not intersect with the first character of the next one, the framework treats
the quantifier as the possessive quantifier because no backtracks can occur.
Possessive quantification is based on loops instead of recursion and works
<P class="backtotop"><A href="#Top">Back to Top</A></P>
<h1><a name="UsageExamples"></a>Usage Examples</h1>
<P>This part on the document illustrates usage of the Harmony regex framework. </P>
<h3><a name="Example1"></a>Example 1</h3>
<p>This example illustrates using the <code>CharSet</code> class, which includes all nodes accepting characters to create a new type of tokens. For that, the class uses the <code>LeafSet</code> class, which is the basic class for tokens. This example shows that the only method you need to override in order to present a new type of tokens is the <code>accept()</code> method. </p>
<pre>class CharSet extends LeafSet {
public int accept(int strIndex, CharSequence testString) {
// checks that the current symbol of the input string is the one this
// instance represents and returns 1 (the number of
// accepted symbols) or -1 if accept fails:
return ( == testString.charAt(strIndex)) ? 1 : -1;
<h3>Example 2</h3>
<p>The following example demonstrates independent implementation of the <code>find()</code> method for <code>SequenceSet</code>.</p>
<p class="note">Note: </p>
<p class="notetext">This changes the find procedure for nodes representing character
sequences and at the same time does not affect the find-match algorithms of
other types of nodes. </p>
<pre>class SequenceSet extends LeafSet {
protected int indexOf(CharSequence str, int from) {
// Boyer-Moore algorithm
public int find(int strIndex, CharSequence testString,
MatchResultImpl matchResult) {
while (strIndex <= strLength) {
// call the fast search method instead of default implementation
strIndex = indexOf(testStr, strIndex);
if (strIndex < 0) {
return -1;
if (next.matches(strIndex + charCount, testString, matchResult) >= 0) {
return strIndex;
return -1;
<h3>Example 3 </h3>
<p>This example illustrates how to turn the match procedure of the <code>.*</code>
patterns into the find procedure, which is potentially faster. </p>
<pre>class DotQuantifierSet extends LeafQuantifierSet {
public int matches(int stringIndex, CharSequence testString,
MatchResultImpl matchResult) {
// find line terminator, since .* represented by this node accepts all characters
// except line terminators
int startSearch = findLineTerminator(stringIndex, strLength, testString);
// run findBack method of the next node, because the previous
// part of the string is accepted by .* and no matter where
// the next part of pattern is found, the procedure works OK.
return next.findBack(stringIndex, startSearch, testString, matchResult);
<P class="backtotop"><A href="#Top">Back to Top</A></P>
<h1><a name="References"></a>References</h1>
<p>[<a name="Fri02"></a>1] Jeffrey E. F. Friedl., <em>Mastering regular expressions</em>,
2nd Edition., July 2002, O'Reilly, ISBN 0-596-00289-0 </p>
<p>[<a name="MY60"></a>2] McNaughton, R. and Yamada, H. <em>Regular Expressions
and State Graphs for Automata</em>, IRA Trans. on Electronic Computers, Vol.
EC-9, No. 1, Mar. 1960, pp 39-47. </p>
<p>[<a name="THO68"></a>3] Thompson, K., <em>Regular Expression search Algorithm</em>,
Communication ACM 11:6 (1968), pp. 419-422. </p>
<p>[<a name="ASU86"></a>4] Alfred V. Aho, Ravi Sethi, Jeffrey
D. Ullman, <em>Compilers, Principles Techniques and Tools</em>, Addison-Wesley
Publishing Company, Inc., 1985, ISBN 0-201-10088-6 </p>
<p>[<a name="JavaSite"></a>5] Java Technology site, <a href="" target="_blank"></a>
<p>[<a name="BM77"></a>6] R. Boyer and S. Moore. A fast string searching algorithm. C. ACM, 20:762-772, 1977. </p>
<p>[<a name="KMP77"></a>7] D.E. Knuth, .I. Morris, and V. Pratt. Fast pattern
matching in strings. SIAM J on Computing, 6:323-350, 1977. </p>
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