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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>
<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>& 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>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>num</code> repetitions</td></tr>
<tr><td><code>p^+num</code></td>
<td>at least <code>num</code> repetitions</td></tr>
<tr><td><code>p^-num</code></td>
<td>at most <code>num</code> repetitions</td></tr>
<tr><td>(<code>name <- p</code>)<sup>+</sup></td> <td>grammar</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>{| p |}</code></td> <td>table capture</td></tr>
<tr><td><code>=name</code></td> <td>back reference</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 -> num</code></td> <td>numbered 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 ~> name</code></td> <td>fold capture
(deprecated)</td></tr>
<tr><td><code>p >> name</code></td> <td>accumulator capture
equivalent to <code>(p % defs[name])</code></td></tr>
</tbody></table>
<p>
Any space appearing in a syntax description can be
replaced by zero or more space characters and Lua-style short 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 and
the index where it ends.
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.gsub (subject, pattern, replacement)</code></h3>
<p>
Does a <em>global substitution</em>,
replacing all occurrences of <code>pattern</code>
in the given <code>subject</code> by <code>replacement</code>.
<h3><code>re.match (subject, pattern)</code></h3>
<p>
Matches the given pattern against the given subject,
returning all captures.
</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 numeral in a string
print(re.find("the number 423 is odd", "[0-9]+")) --> 12 14
-- returns all words in a string
print(re.match("the number 423 is odd", "({%a+} / .)*"))
--> the number is odd
-- returns the first numeral in a string
print(re.match("the number 423 is odd", "s <- {%d+} / . s"))
--> 423
-- substitutes a dot for each vowel in a string
print(re.gsub("hello World", "[aeiou]", "."))
--> h.ll. W.rld
</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)
close <- ']' =eq ']' / . close
]])
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 (grammar / alternative)
alternative <- seq ('/' S seq)*
seq <- prefix*
prefix <- '&' S prefix / '!' S prefix / suffix
suffix <- primary S (([+*?]
/ '^' [+-]? num
/ '->' S (string / '{}' / name)
/ '>>' S name
/ '=>' S name) S)*
primary <- '(' exp ')' / string / class / defined
/ '{:' (name ':')? exp ':}'
/ '=' name
/ '{}'
/ '{~' exp '~}'
/ '{|' 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>
This module is part of the <a href="lpeg.html">LPeg</a> package and shares
its <a href="lpeg.html#license">license</a>.
</div> <!-- id="content" -->
</div> <!-- id="main" -->
</div> <!-- id="container" -->
</body>
</html>