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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<title>LPeg.re - Regex syntax for LPEG</title>
<link rel="stylesheet"
href="http://www.inf.puc-rio.br/~roberto/lpeg/doc.css"
type="text/css"/>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
</head>
<body>
<!-- $Id: re.html,v 1.17 2011/01/10 15:08:06 roberto Exp $ -->
<div id="container">
<div id="product">
<div id="product_logo">
<a href="http://www.inf.puc-rio.br/~roberto/lpeg/">
<img alt="LPeg logo" src="lpeg-128.gif"/>
</a>
</div>
<div id="product_name"><big><strong>LPeg.re</strong></big></div>
<div id="product_description">
Regex syntax for LPEG
</div>
</div> <!-- id="product" -->
<div id="main">
<div id="navigation">
<h1>re</h1>
<ul>
<li><a href="#basic">Basic Constructions</a></li>
<li><a href="#func">Functions</a></li>
<li><a href="#ex">Some Examples</a></li>
<li><a href="#license">License</a></li>
</ul>
</li>
</ul>
</div> <!-- id="navigation" -->
<div id="content">
<h2><a name="basic"></a>The <code>re</code> Module</h2>
<p>
The <code>re</code> module
(provided by file <code>re.lua</code> in the distribution)
supports a somewhat conventional regex syntax
for pattern usage within <a href="lpeg.html">LPeg</a>.
</p>
<p>
The next table summarizes <code>re</code>'s syntax.
A <code>p</code> represents an arbitrary pattern;
<code>num</code> represents a number (<code>[0-9]+</code>);
<code>name</code> represents an identifier
(<code>[a-zA-Z][a-zA-Z0-9_]*</code>).
Constructions are listed in order of decreasing precedence.
<table border="1">
<tbody><tr><td><b>Syntax</b></td><td><b>Description</b></td></tr>
<tr><td><code>( p )</code></td> <td>grouping</td></tr>
<tr><td><code>'string'</code></td> <td>literal string</td></tr>
<tr><td><code>"string"</code></td> <td>literal string</td></tr>
<tr><td><code>[class]</code></td> <td>character class</td></tr>
<tr><td><code>.</code></td> <td>any character</td></tr>
<tr><td><code>%name</code></td>
<td>pattern <code>defs[name]</code> or a pre-defined pattern</td></tr>
<tr><td><code>name</code></td><td>non terminal</td></tr>
<tr><td><code><name></code></td><td>non terminal</td></tr>
<tr><td><code>{}</code></td> <td>position capture</td></tr>
<tr><td><code>{ p }</code></td> <td>simple capture</td></tr>
<tr><td><code>{: p :}</code></td> <td>anonymous group capture</td></tr>
<tr><td><code>{:name: p :}</code></td> <td>named group capture</td></tr>
<tr><td><code>{~ p ~}</code></td> <td>substitution capture</td></tr>
<tr><td><code>=name</code></td> <td>back reference
</td></tr>
<tr><td><code>p ?</code></td> <td>optional match</td></tr>
<tr><td><code>p *</code></td> <td>zero or more repetitions</td></tr>
<tr><td><code>p +</code></td> <td>one or more repetitions</td></tr>
<tr><td><code>p^num</code></td> <td>exactly <code>n</code> repetitions</td></tr>
<tr><td><code>p^+num</code></td>
<td>at least <code>n</code> repetitions</td></tr>
<tr><td><code>p^-num</code></td>
<td>at most <code>n</code> repetitions</td></tr>
<tr><td><code>p -> 'string'</code></td> <td>string capture</td></tr>
<tr><td><code>p -> "string"</code></td> <td>string capture</td></tr>
<tr><td><code>p -> {}</code></td> <td>table capture</td></tr>
<tr><td><code>p -> name</code></td> <td>function/query/string capture
equivalent to <code>p / defs[name]</code></td></tr>
<tr><td><code>p => name</code></td> <td>match-time capture
equivalent to <code>lpeg.Cmt(p, defs[name])</code></td></tr>
<tr><td><code>& p</code></td> <td>and predicate</td></tr>
<tr><td><code>! p</code></td> <td>not predicate</td></tr>
<tr><td><code>p1 p2</code></td> <td>concatenation</td></tr>
<tr><td><code>p1 / p2</code></td> <td>ordered choice</td></tr>
<tr><td>(<code>name <- p</code>)<sup>+</sup></td> <td>grammar</td></tr>
</tbody></table>
<p>
Any space appearing in a syntax description can be
replaced by zero or more space characters and Lua-style comments
(<code>--</code> until end of line).
</p>
<p>
Character classes define sets of characters.
An initial <code>^</code> complements the resulting set.
A range <em>x</em><code>-</code><em>y</em> includes in the set
all characters with codes between the codes of <em>x</em> and <em>y</em>.
A pre-defined class <code>%</code><em>name</em> includes all
characters of that class.
A simple character includes itself in the set.
The only special characters inside a class are <code>^</code>
(special only if it is the first character);
<code>]</code>
(can be included in the set as the first character,
after the optional <code>^</code>);
<code>%</code> (special only if followed by a letter);
and <code>-</code>
(can be included in the set as the first or the last character).
</p>
<p>
Currently the pre-defined classes are similar to those from the
Lua's string library
(<code>%a</code> for letters,
<code>%A</code> for non letters, etc.).
There is also a class <code>%nl</code>
containing only the newline character,
which is particularly handy for grammars written inside long strings,
as long strings do not interpret escape sequences like <code>\n</code>.
</p>
<h2><a name="func">Functions</a></h2>
<h3><code>re.compile (string, [, defs])</code></h3>
<p>
Compiles the given string and
returns an equivalent LPeg pattern.
The given string may define either an expression or a grammar.
The optional <code>defs</code> table provides extra Lua values
to be used by the pattern.
</p>
<h3><code>re.find (subject, pattern [, init])</code></h3>
<p>
Searches the given pattern in the given subject.
If it finds a match,
returns the index where this occurrence starts,
plus the captures made by the pattern (if any).
Otherwise, returns nil.
</p>
<p>
An optional numeric argument <code>init</code> makes the search
starts at that position in the subject string.
As usual in Lua libraries,
a negative value counts from the end.
</p>
<h3><code>re.match (subject, pattern)</code></h3>
<p>
Matches the given pattern against the given subject.
</p>
<h3><code>re.updatelocale ()</code></h3>
<p>
Updates the pre-defined character classes to the current locale.
</p>
<h2><a name="ex">Some Examples</a></h2>
<h3>A complete simple program</h3>
<p>
The next code shows a simple complete Lua program using
the <code>re</code> module:
</p>
<pre class="example">
local re = require"re"
-- find the position of the first number in a string
print(re.find("the number 423 is odd", "[0-9]+")) --> 12
-- similar, but also captures (and returns) the number
print(re.find("the number 423 is odd", "{[0-9]+}")) --> 12 423
-- returns all words in a string
print(re.match("the number 423 is odd", "({%a+} / .)*"))
--> the number is odd
</pre>
<h3>Balanced parentheses</h3>
<p>
The following call will produce the same pattern produced by the
Lua expression in the
<a href="lpeg.html#balanced">balanced parentheses</a> example:
</p>
<pre class="example">
b = re.compile[[ balanced <- "(" ([^()] / balanced)* ")" ]]
</pre>
<h3>String reversal</h3>
<p>
The next example reverses a string:
</p>
<pre class="example">
rev = re.compile[[ R <- (!.) -> '' / ({.} R) -> '%2%1']]
print(rev:match"0123456789") --> 9876543210
</pre>
<h3>CSV decoder</h3>
<p>
The next example replicates the <a href="lpeg.html#CSV">CSV decoder</a>:
</p>
<pre class="example">
record = re.compile[[
record <- ( field (',' field)* ) -> {} (%nl / !.)
field <- escaped / nonescaped
nonescaped <- { [^,"%nl]* }
escaped <- '"' {~ ([^"] / '""' -> '"')* ~} '"'
]]
</pre>
<h3>Lua's long strings</h3>
<p>
The next example matches Lua long strings:
</p>
<pre class="example">
c = re.compile([[
longstring <- ('[' {:eq: '='* :} '[' close) -> void
close <- ']' =eq ']' / . close
]], {void = function () end})
print(c:match'[==[]]===]]]]==]===[]') --> 17
</pre>
<h3>Abstract Syntax Trees</h3>
<p>
This example shows a simple way to build an
abstract syntax tree (AST) for a given grammar.
To keep our example simple,
let us consider the following grammar
for lists of names:
</p>
<pre class="example">
p = re.compile[[
listname <- (name s)*
name <- [a-z][a-z]*
s <- %s*
]]
</pre>
<p>
Now, we will add captures to build a corresponding AST.
As a first step, the pattern will build a table to
represent each non terminal;
terminals will be represented by their corresponding strings:
</p>
<pre class="example">
c = re.compile[[
listname <- (name s)* -> {}
name <- {[a-z][a-z]*} -> {}
s <- %s*
]]
</pre>
<p>
Now, a match against <code>"hi hello bye"</code>
results in the table
<code>{{"hi"}, {"hello"}, {"bye"}}</code>.
</p>
<p>
For such a simple grammar,
this AST is more than enough;
actually, the tables around each single name
are already overkilling.
More complex grammars,
however, may need some more structure.
Specifically,
it would be useful if each table had
a <code>tag</code> field telling what non terminal
that table represents.
We can add such a tag using
<a href="lpeg.html/#cap-g">named group captures</a>:
</p>
<pre class="example">
x = re.compile[[
listname <- ({:tag: '' -> 'list':} (name s)*) -> {}
name <- ({:tag: '' -> 'id':} {[a-z][a-z]*}) -> {}
s <- ' '*
]]
</pre>
<p>
With these group captures,
a match against <code>"hi hello bye"</code>
results in the following table:
</p>
<pre class="example">
{tag="list",
{tag="id", "hi"},
{tag="id", "hello"},
{tag="id", "bye"}
}
</pre>
<h3>Indented blocks</h3>
<p>
This example breaks indented blocks into tables,
respecting the indentation:
</p>
<pre class="example">
p = re.compile[[
block <- ({:ident:' '*:} line
((=ident !' ' line) / &(=ident ' ') block)*) -> {}
line <- {[^%nl]*} %nl
]]
</pre>
<p>
As an example,
consider the following text:
</p>
<pre class="example">
t = p:match[[
first line
subline 1
subline 2
second line
third line
subline 3.1
subline 3.1.1
subline 3.2
]]
</pre>
<p>
The resulting table <code>t</code> will be like this:
</p>
<pre class="example">
{'first line'; {'subline 1'; 'subline 2'; ident = ' '};
'second line';
'third line'; { 'subline 3.1'; {'subline 3.1.1'; ident = ' '};
'subline 3.2'; ident = ' '};
ident = ''}
</pre>
<h3>Macro expander</h3>
<p>
This example implements a simple macro expander.
Macros must be defined as part of the pattern,
following some simple rules:
</p>
<pre class="example">
p = re.compile[[
text <- {~ item* ~}
item <- macro / [^()] / '(' item* ')'
arg <- ' '* {~ (!',' item)* ~}
args <- '(' arg (',' arg)* ')'
-- now we define some macros
macro <- ('apply' args) -> '%1(%2)'
/ ('add' args) -> '%1 + %2'
/ ('mul' args) -> '%1 * %2'
]]
print(p:match"add(mul(a,b), apply(f,x))") --> a * b + f(x)
</pre>
<p>
A <code>text</code> is a sequence of items,
wherein we apply a substitution capture to expand any macros.
An <code>item</code> is either a macro,
any character different from parentheses,
or a parenthesized expression.
A macro argument (<code>arg</code>) is a sequence
of items different from a comma.
(Note that a comma may appear inside an item,
e.g., inside a parenthesized expression.)
Again we do a substitution capture to expand any macro
in the argument before expanding the outer macro.
<code>args</code> is a list of arguments separated by commas.
Finally we define the macros.
Each macro is a string substitution;
it replaces the macro name and its arguments by its corresponding string,
with each <code>%</code><em>n</em> replaced by the <em>n</em>-th argument.
</p>
<h3>Patterns</h3>
<p>
This example shows the complete syntax
of patterns accepted by <code>re</code>.
</p>
<pre class="example">
p = [=[
pattern <- exp !.
exp <- S (alternative / grammar)
alternative <- seq ('/' S seq)*
seq <- prefix*
prefix <- '&' S prefix / '!' S prefix / suffix
suffix <- primary S (([+*?]
/ '^' [+-]? num
/ '->' S (string / '{}' / name)
/ '=>' S name) S)*
primary <- '(' exp ')' / string / class / defined
/ '{:' (name ':')? exp ':}'
/ '=' name
/ '{}'
/ '{~' exp '~}'
/ '{' exp '}'
/ '.'
/ name S !arrow
/ '<' name '>' -- old-style non terminals
grammar <- definition+
definition <- name S arrow exp
class <- '[' '^'? item (!']' item)* ']'
item <- defined / range / .
range <- . '-' [^]]
S <- (%s / '--' [^%nl]*)* -- spaces and comments
name <- [A-Za-z][A-Za-z0-9_]*
arrow <- '<-'
num <- [0-9]+
string <- '"' [^"]* '"' / "'" [^']* "'"
defined <- '%' name
]=]
print(re.match(p, p)) -- a self description must match itself
</pre>
<h2><a name="license">License</a></h2>
<p>
Copyright © 2008-2010 Lua.org, PUC-Rio.
</p>
<p>
Permission is hereby granted, free of charge,
to any person obtaining a copy of this software and
associated documentation files (the "Software"),
to deal in the Software without restriction,
including without limitation the rights to use,
copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software,
and to permit persons to whom the Software is
furnished to do so,
subject to the following conditions:
</p>
<p>
The above copyright notice and this permission notice
shall be included in all copies or substantial portions of the Software.
</p>
<p>
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
</p>
</div> <!-- id="content" -->
</div> <!-- id="main" -->
<div id="about">
<p><small>
$Id: re.html,v 1.17 2011/01/10 15:08:06 roberto Exp $
</small></p>
</div> <!-- id="about" -->
</div> <!-- id="container" -->
</body>
</html>