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    <title>LPeg.re - Regex syntax for LPEG</title>
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  <div id="product_name"><big><strong>LPeg.re</strong></big></div>
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     Regex syntax for LPEG
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<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>
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<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>&amp; 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 &lt;- 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>&lt;name&gt;</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 -&gt; 'string'</code></td> <td>string capture</td></tr>
<tr><td><code>p -&gt; "string"</code></td> <td>string capture</td></tr>
<tr><td><code>p -&gt; num</code></td> <td>numbered capture</td></tr>
<tr><td><code>p -&gt; name</code></td> <td>function/query/string capture
equivalent to <code>p / defs[name]</code></td></tr>
<tr><td><code>p =&gt; name</code></td> <td>match-time capture
equivalent to <code>lpeg.Cmt(p, defs[name])</code></td></tr>
<tr><td><code>p ~&gt; name</code></td> <td>fold capture
(deprecated)</td></tr>
<tr><td><code>p &gt;&gt; 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]+"))  --&gt; 12    14

-- returns all words in a string
print(re.match("the number 423 is odd", "({%a+} / .)*"))
--&gt; the    number    is    odd

-- returns the first numeral in a string
print(re.match("the number 423 is odd", "s &lt;- {%d+} / . s"))
--&gt; 423

-- substitutes a dot for each vowel in a string
print(re.gsub("hello World", "[aeiou]", "."))
--&gt; 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 &lt;- "(" ([^()] / balanced)* ")"  ]]
</pre>

<h3>String reversal</h3>
<p>
The next example reverses a string:
</p>
<pre class="example">
rev = re.compile[[ R &lt;- (!.) -&gt; '' / ({.} R) -&gt; '%2%1']]
print(rev:match"0123456789")   --&gt; 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 &lt;- {| field (',' field)* |} (%nl / !.)
  field &lt;- escaped / nonescaped
  nonescaped &lt;- { [^,"%nl]* }
  escaped &lt;- '"' {~ ([^"] / '""' -&gt; '"')* ~} '"'
]]
</pre>

<h3>Lua's long strings</h3>
<p>
The next example matches Lua long strings:
</p>
<pre class="example">
c = re.compile([[
  longstring &lt;- ('[' {:eq: '='* :} '[' close)
  close &lt;- ']' =eq ']' / . close
]])

print(c:match'[==[]]===]]]]==]===[]')   --&gt; 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 &lt;- (name s)*
      name &lt;- [a-z][a-z]*
      s &lt;- %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 &lt;- {| (name s)* |}
      name &lt;- {| {[a-z][a-z]*} |}
      s &lt;- %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 &lt;- {| {:ident:' '*:} line
           ((=ident !' ' line) / &amp;(=ident ' ') block)* |}
  line &lt;- {[^%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 &lt;- {~ item* ~}
      item &lt;- macro / [^()] / '(' item* ')'
      arg &lt;- ' '* {~ (!',' item)* ~}
      args &lt;- '(' arg (',' arg)* ')'
      -- now we define some macros
      macro &lt;- ('apply' args) -&gt; '%1(%2)'
             / ('add' args) -&gt; '%1 + %2'
             / ('mul' args) -&gt; '%1 * %2'
]]

print(p:match"add(mul(a,b), apply(f,x))")   --&gt; 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         &lt;- exp !.
exp             &lt;- S (grammar / alternative)

alternative     &lt;- seq ('/' S seq)*
seq             &lt;- prefix*
prefix          &lt;- '&amp;' S prefix / '!' S prefix / suffix
suffix          &lt;- primary S (([+*?]
                            / '^' [+-]? num
                            / '-&gt;' S (string / '{}' / name)
                            / '&gt&gt;' S name
                            / '=&gt;' S name) S)*

primary         &lt;- '(' exp ')' / string / class / defined
                 / '{:' (name ':')? exp ':}'
                 / '=' name
                 / '{}'
                 / '{~' exp '~}'
                 / '{|' exp '|}'
                 / '{' exp '}'
                 / '.'
                 / name S !arrow
                 / '&lt;' name '&gt;'          -- old-style non terminals

grammar         &lt;- definition+
definition      &lt;- name S arrow exp

class           &lt;- '[' '^'? item (!']' item)* ']'
item            &lt;- defined / range / .
range           &lt;- . '-' [^]]

S               &lt;- (%s / '--' [^%nl]*)*   -- spaces and comments
name            &lt;- [A-Za-z_][A-Za-z0-9_]*
arrow           &lt;- '&lt;-'
num             &lt;- [0-9]+
string          &lt;- '"' [^"]* '"' / "'" [^']* "'"
defined         &lt;- '%' 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>.


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