## Introduction

*LPeg* is a new pattern-matching library for Lua,
based on
Parsing Expression Grammars (PEGs).
In this text, I assume you are familiar with PEGs.
If you are not, you can get a quick start reading
the
Wikipedia Entry for PEGs
or Section 2 of
Parsing Expression Grammars: A Recognition-Based Syntactic Foundation
(the section has only one page).
The nice thing about PEGs is that it has a formal basis
(instead of being an ad-hoc set of features),
allows an *efficient and simple implementation*,
and does most things we expect from a pattern-matching library
(and more, as we can define entire grammars).

Following the Snobol tradition,
LPeg defines patterns as first-class objects.
That is, patterns are regular Lua values
(represented by userdata).
The library offers several functions to create
and compose patterns.
With the use of metamethods,
several of these functions are provided as infix or prefix
operators.
On the one hand,
the result is usually much more verbose than the typical
encoding of patterns using the so called
*regular expressions*
(which typically are not regular expressions in the formal sense).
On the other hand,
first-class patterns allow much better documentation
(as it is easy to comment the code,
to use auxiliary variables to break complex definitions, etc.)
and are extensible,
as we can define new functions to create and compose patterns.

For a quick glance of the library, the following table summarizes its basic operations for creating patterns:

Operator | Description |

`lpeg.P(string)` |
Matches `string` literally |

`lpeg.P(number)` |
Matches exactly `number` characters |

`lpeg.S(string)` |
Matches any character in `string` (set) |

`lpeg.R("` |
Matches any character between x and y (range) |

`patt^n` |
Matches at least `n` repetitions of `patt` |

`patt^-n` |
Matches at most `n` repetitions of `patt` |

`patt1 * patt2` |
Matches `patt1` followed by `patt2` |

`patt1 + patt2` |
Matches `patt1` or `patt2`
(ordered choice) |

`patt1 - patt2` |
Matches `patt1` if `patt2` does not match |

`-patt` |
Equivalent to `"" - patt` |

`#patt` |
Matches `patt` but consumes no input |

As a very simple example, `lpeg.R("09")^1`

matches
a non-empty sequence of digits.
As a not so simple example,
`-lpeg.P(1)`

(which can be written as `lpeg.P(-1)`

or simply `-1`

for operations expecting a pattern)
matches an empty string only if it cannot match a single character;
so, it succeeds only at the subject's end.

Those not convinced by the previous syntax
can try the `re`

module,
which implements patterns following a regular-expression style
(e.g., `[09]+`

).
(This module is 200 lines of Lua code,
and of course uses LPeg to parse regular expressions.)

## Functions

`lpeg.match (pattern, subject [, init])`

The matching function. It attempts to match the given pattern against the subject string. If the match succeeds, returns the index in the subject of the first character after the match, or the values of captured values (if the pattern captured any value).

An optional numeric argument `init`

makes the match
starts at that position in the subject string.
As usual in Lua libraries,
a negative value counts from the end.

Unlike typical pattern-matching functions,
`match`

works only in *anchored* mode;
that is, it tries to match the pattern with a prefix of
the given subject string (at position `init`

),
not with an arbitrary substring of the subject.
So, if we want to find a pattern anywhere in a string,
we must either write a loop in Lua or write a pattern that
matches anywhere.
This second approach is easy and quite efficient;
see examples.

`lpeg.type (value)`

If the given value is a pattern,
returns the string `"pattern"`

.
Otherwise returns nil.

`lpeg.version ()`

Returns a string with the running version of LPEG.

## Basic Constructions

The following operations build patterns.
All operations that expect a pattern as an argument
may receive also strings, tables, numbers, booleans, or functions,
which are translated to patterns according to
the rules of function `lpeg.P`

.

`lpeg.P (value)`

Converts the given value into a proper pattern, according to the following rules:

If the argument is a pattern, it is returned unmodified.

If the argument is a string, it is translated to a pattern that matches literally the string.

If the argument is a number, it is translated as follows. A non-negative number

*n*gives a pattern that matches exactly*n*characters; a negative number*-n*gives a pattern that succeeds only if the input string does not have*n*characters. It is (as expected) equivalent to the unary minus operation (see below) applied over the absolute value of*n*.If the argument is a boolean, the result is a pattern that always succeeds or always fails (according to the boolean value), without consuming any input.

If the argument is a table, it is interpreted as a grammar (see Grammars).

If the argument is a function, returns a pattern equivalent to a match-time capture over the empty string.

If the function is called with parameters

*s*and*i*, its result is valid if it is in the range*[i, len(s) + 1]*.

`lpeg.R ({range})`

Returns a pattern that matches any single character
belonging to one of the given *ranges*.
Each `range`

is a string *xy* of length 2,
representing all characters with code
between the codes of *x* and *y*
(both inclusive).

As an example, the pattern
`lpeg.R("09")`

matches any digit,
and `lpeg.R("az", "AZ")`

matches any ASCII letter.

`lpeg.S (string)`

Returns a pattern that matches any single character that
appears in the given string.
(The `S`

stands for *Set*.)

As an example, the pattern
`lpeg.S("+-*/")`

matches any arithmetic operator.

Note that, if `s`

is a character
(that is, a string of length 1),
then `lpeg.P(s)`

is equivalent to `lpeg.S(s)`

which is equivalent to `lpeg.R(s..s)`

.
Note also that both `lpeg.S("")`

and `lpeg.R()`

are patterns that always fail.

`lpeg.V (v)`

This operation creates a non-terminal (a *variable*)
for a grammar.
The created non-terminal refers to the rule indexed by `v`

in the enclosing grammar.
(See Grammars for details.)

`#patt`

Returns a pattern equivalent to *&patt* in the original
PEG notation.
This is a pattern that matches only if the input string
does match `patt`

,
but without consuming any input,
independently of success or failure.

When it succeeds,
`#patt`

produces all captures produced by `patt`

.

`-patt`

Returns a pattern equivalent to *!patt* in the original
PEG notation.
This pattern matches only if the input string
does not match `patt`

.
It does not consume any input,
independently of success or failure.

As an example, the pattern
`-1`

matches only the end of string.

This pattern never produces any captures,
because either `patt`

fails
or `-patt`

fails.
(A failing pattern produces no captures.)

`patt1 + patt2`

Returns a pattern equivalent to an *ordered choice*
of `patt1`

and `patt2`

.
(This is denoted by *patt1 / patt2* in the original PEG notation,
not to be confused with the `/`

operation in LPeg.)
It matches either `patt1`

or `patt2`

(with no backtracking once one of them succeeds).
The identity element for this operation is the pattern
`lpeg.P(false)`

,
which always fails.

If both `patt1`

and `patt2`

are
character sets,
this operation is equivalent to set union:

lower = lpeg.R("az") upper = lpeg.R("AZ") letter = lower + upper

`patt1 - patt2`

Returns a pattern equivalent to *!patt2 patt1*.
This pattern asserts that the input does not match
`patt2`

and then matches `patt1`

.

If both `patt1`

and `patt2`

are
character sets,
this operation is equivalent to set difference.
Note that `-patt`

is equivalent to `"" - patt`

(or `0 - patt`

).
If `patt`

is a character set,
`1 - patt`

is its complement.

`patt1 * patt2`

Returns a pattern that matches `patt1`

and then matches `patt2`

,
starting where `patt1`

finished.
The identity element for this operation is the
pattern `lpeg.P(true)`

,
which always succeeds.

(LPeg uses the `*`

operator
[instead of the more obvious `..`

]
both because it has
the right priority and because in formal languages it is
common to use a dot for denoting concatenation.)

`patt^n`

If `n`

is nonnegative,
this pattern is
equivalent to *patt ^{n} patt**.
It matches at least

`n`

occurrences of `patt`

.
Otherwise, when `n`

is negative,
this pattern is equivalent to *(patt?) ^{-n}*.
That is, it matches at most

`-n`

occurrences of `patt`

.
In particular, `patt^0`

is equivalent to *patt**,
`patt^1`

is equivalent to *patt+*,
and `patt^-1`

is equivalent to *patt?*
in the original PEG notation.

In all cases,
the resulting pattern is greedy with no backtracking.
That is, it matches only the longest possible sequence
of matches for `patt`

.

## Grammars

With the use of Lua variables, it is possible to define patterns incrementally, with each new pattern using previously defined ones. However, this technique does not allow the definition of recursive patterns. For recursive patterns, we need real grammars.

LPeg represents grammars with tables, where each entry is a rule.

The call `lpeg.V(v)`

creates a pattern that represents the nonterminal
(or *variable*) with index `v`

in a grammar.
Because the grammar still does not exist when
this function is evaluated,
the result is an *open reference* to the respective rule.

A table is *fixed* when it is converted to a pattern
(either by calling `lpeg.P`

or by using it wherein a
pattern is expected).
Then every open reference created by `lpeg.V(v)`

is corrected to refer to the rule indexed by `v`

in the table.

When a table is fixed,
the result is a pattern that matches its *initial rule*.
The entry with index 1 in the table defines its initial rule.
If that entry is a string,
it is assumed to be the name of the initial rule.
Otherwise, LPeg assumes that the entry 1 itself is the initial rule.

As an example, the following grammar matches strings of a's and b's that have the same number of a's and b's:

equalcount = lpeg.P{ "S"; -- initial rule name S = "a" * lpeg.V"B" + "b" * lpeg.V"A" + "", A = "a" * lpeg.V"S" + "b" * lpeg.V"A" * lpeg.V"A", B = "b" * lpeg.V"S" + "a" * lpeg.V"B" * lpeg.V"B", } * -1

## Captures

Captures specify what a match operation should return
(the so called *semantic information*).
LPeg offers several kinds of captures,
which produces values based on matches and combine them to
produce new values.

The following table summarizes the basic captures:

Operation | What it Produces |

`lpeg.C(patt)` |
the match for `patt` |

`lpeg.Cc(values)` |
the given values (matches the empty string) |

`lpeg.Cp()` |
the current position (matches the empty string) |

`lpeg.Cb(n)` |
the value produced by the n^{th} previous
capture (matches the empty string) |

`lpeg.Carg(n)` |
the value of the n^{th} extra argument to
`lpeg.match` (matches the empty string) |

`lpeg.Cs(patt)` |
the match for `patt`
with the values from nested captures replacing their matches |

`lpeg.Ct(patt)` |
a table with all captures from `patt` |

`lpeg.Ca(patt)` |
an accumulation (or folding) of the
captures from `patt` |

`patt / string` |
`string` , with some marks replaced by captures
of `patt` |

`patt / table` |
`table[c]` , where `c` is the (first)
capture of `patt` |

`patt / function` |
the returns of `function` applied to the captures
of `patt` |

`lpeg.Cmt(patt, function)` |
(see match-time capture) |

A capture pattern produces its values every time it succeeds.
For instance,
a capture inside a loop produces as many values as matched by the loop.
A capture produces a value only when it succeeds.
For instance,
the pattern `lpeg.C(lpeg.P"a"^-1)`

produces the empty string when there is no `"a"`

(because the pattern `"a"?`

succeeds),
while the pattern `lpeg.C("a")^-1`

does not produce any value when there is no `"a"`

(because the pattern `"a"`

fails).

Usually,
LPEG evaluates all captures only after (and if) the entire match succeeds.
At *match time* it only gathers enough information
to produce the capture values later.
As a particularly important consequence,
most captures cannot affect the way a pattern matches a subject.
The only exception to this rule is the
so-called *match-time capture*.
When a match-time capture matches,
it forces the immediate evaluation of all its nested captures
and then calls its corresponding function,
which tells whether the match succeeds and also
what values are produced.

`lpeg.C (patt)`

Creates a *simple capture*,
which captures the substring of the subject that matches `patt`

.
The captured value is a string.
If `patt`

has other captures,
their values are returned after this one.

`lpeg.Ca (patt)`

Creates an *accumulator capture*.
This capture assumes that `patt`

should produce
at least one captured value of any kind,
which becomes the initial value of an *accumulator*.
Pattern `patt`

then may produce
zero or more *function captures*.
Each of these functions in these captures is called having the
accumulator as its first argument
(followed by any other arguments provided by its own pattern),
and the value returned by the function becomes the new value
of the accumulator.
The final value of this accumulator is the sole result of
the whole capture.

As an example, the following pattern matches a list of numbers separated by commas and returns their addition:

-- matches a numeral and captures its value local number = lpeg.R"09"^1 / tonumber -- auxiliary function to add two numbers local function add (acc, newvalue) return acc + newvalue end list = lpeg.Ca(number * ("," * number / add)^0) -- example of use print(list:match("10,30,43")) --> 83

`lpeg.Carg (n)`

Creates an *argument capture*.
This pattern matches the empty string and
produces the value given as the n^{th} extra
argument given in the call to `lpeg.match`

.

`lpeg.Cb (n)`

Creates a *back capture*.
This pattern matches the empty string and
produces the values produced by the n^{th} previous capture.

Captures are numbered dynamically. So, the first previous capture is the last capture to match before the current one. The numbering includes only complete captures; so, if the back capture is inside another capture, this enclosing capture is ignored (because it is not complete when the back capture is seen). Numbering does not count nested captures. Numbering counts captures, not the values produced by them; it does not matter whether a capture produces zero or many values, it counts as one.

*
This is an experimental feature.
It probably will be changed or even removed in future releases.
*

`lpeg.Cc ({value})`

Creates a *constant capture*.
This pattern matches the empty string and
produces all given values as its captured values.

`lpeg.Cp ()`

Creates a *position capture*.
It matches the empty string and
captures the position in the subject where the match occurs.
The captured value is a number.

`lpeg.Cs (patt)`

Creates a *substitution capture*,
which captures the substring of the subject that matches `patt`

,
with *substitutions*.
For any capture inside `patt`

,
the substring that matched the capture is replaced by the capture value
(which should be a string).
The capture values from `patt`

are not returned independently
(only as substrings in the resulting string).

`lpeg.Ct (patt)`

Creates a *table capture*.
This capture creates a table and puts all captures made by
`patt`

inside this table in successive integer keys,
starting at 1.

The captured value is only this table.
The captures made by `patt`

are not
returned independently (only as table elements).

`patt / string`

Creates a *string capture*.
It creates a capture string based on `string`

.
The captured value is a copy of `string`

,
except that the character `%`

works as an escape character:
any sequence in `string`

of the form `%`

,
with *n**n* between 1 and 9,
stands for the match of the *n*-th capture in `patt`

.
(Currently these nested captures can be only simple captures.)
The sequence `%0`

stands for the whole match.
The sequence `%%`

stands for a single `%`

.

`patt / table`

Creates a *query capture*.
It indexes the given table using as key the value of the first capture
of `patt`

,
or the whole match if `patt`

made no capture.
The value at that index is the final value of the capture.
If the table does not have that key,
there is no captured value.
Everything works as if there was no capture.

`patt / function`

Creates a *function capture*.
It calls the given function passing all captures made by
`patt`

as arguments,
or the whole match if `patt`

made no capture.
The values returned by the function
are the final values of the capture.
(This capture may create multiple values.)
In particular,
if `function`

returns no value,
there is no captured value;
everything works as if there was no capture.

`lpeg.Cmt(patt, function)`

Creates a *match-time capture*.
Unlike all other captures,
this one is evaluated immediately when a match occurs.
It forces the immediate evaluation of all its nested captures
and then calls `function`

.

The function gets as arguments the entire subject,
the current position (after the match of `patt`

),
plus any capture values produced by `patt`

.

The first value returned by `function`

defines how the match happens.
If the call returns a number,
the match succeeds
and the returned number becomes the new current position.
(Assuming a subject *s* and current position *i*,
the returned number must be in the range *[i, len(s) + 1]*.)
If the call returns **false**, **nil**, or no value,
the match fails.

Any extra values returned by the function become the values produced by the capture.

## Some Examples

### Splitting a String

The following code splits a string using a given pattern
`sep`

as a separator:

function split (s, sep) sep = lpeg.P(sep) local elem = lpeg.C((1 - sep)^0) local p = elem * (sep * elem)^0 return lpeg.match(p, s) end

First the function ensures that `sep`

is a proper pattern.
The pattern `elem`

is a repetition of zero of more
arbitrary characters as long as there is not a match against
the separator. It also captures its result.
The pattern `p`

matches a list of elements separated
by `sep`

.

If the split results in too many values, it may overflow the maximum number of values that can be returned by a Lua function. In this case, we should collect these values in a table:

function split (s, sep) sep = lpeg.P(sep) local elem = lpeg.C((1 - sep)^0) local p = lpeg.Ct(elem * (sep * elem)^0) -- make a table capture return lpeg.match(p, s) end

### Searching for a Pattern

The primitive `match`

works only in anchored mode.
If we want to find a pattern anywhere in a string,
we must write a pattern that matches anywhere.

Because patterns are composable,
we can write a function that,
given any arbitrary pattern `p`

,
returns a new pattern that searches for `p`

anywhere in a string.
There are several ways to do the search.
One way is like this:

function anywhere (p) return lpeg.P{ p + 1 * lpeg.V(1) } end

This grammar has a straight reading:
it matches `p`

or skips one character and tries again.

If we want to know where the pattern is in the string (instead of knowing only that it is there somewhere), we can add position captures to the pattern:

local I = lpeg.Cp() function anywhere (p) return lpeg.P{ I * p * I + 1 * lpeg.V(1) } end

Another option for the search is like this:

local I = lpeg.Cp() function anywhere (p) return (1 - lpeg.P(p))^0 * I * p * I end

Again the pattern has a straight reading:
it skips as many characters as possible while not matching `p`

,
and then matches `p`

(plus appropriate captures).

If we want to look for a pattern only at word boundaries, we can use the following transformer:

local wordletter = lpeg.R("AZ", "az") function atwordboundary (p) return lpeg.P{ [1] = p + wordletter^0 * (1 - wordletter)^1 * lpeg.V(1) } end

### Balanced Parentheses

The following pattern matches only strings with balanced parentheses:

b = lpeg.P{ "(" * ((1 - lpeg.S"()") + lpeg.V(1))^0 * ")" }

Reading the first (and only) rule of the given grammar,
we have that a balanced string is
an open parenthesis,
followed by zero or more repetitions of either
a non-parenthesis character or
a balanced string (`lpeg.V(1)`

),
followed by a closing parenthesis.

### Global Substitution

The next example does a job somewhat similar to `string.gsub`

.
It receives a pattern and a replacement value,
and substitutes the replacement value for all occurrences of the pattern
in a given string:

function gsub (s, patt, repl) patt = lpeg.P(patt) patt = lpeg.Cs((patt / repl + 1)^0) return lpeg.match(patt, s) end

As in `string.gsub`

,
the replacement value can be a string,
a function, or a table.

### Comma-Separated Values (CSV)

This example breaks a string into comma-separated values, returning all fields:

local field = '"' * lpeg.Cs(((lpeg.P(1) - '"') + lpeg.P'""' / '"')^0) * '"' + lpeg.C((1 - lpeg.S',\n"')^0) local record = field * (',' * field)^0 * (lpeg.P'\n' + -1) function csv (s) return lpeg.match(record, s) end

A field is either a quoted field (which may contain any character except an individual quote, which may be written as two quotes that are replaced by one) or an unquoted field (which cannot contain commas, newlines, or quotes). A record is a list of fields separated by commas, ending with a newline or the string end (-1).

### UTF-8 and Latin 1

It is not difficult to use LPeg to convert a string from utf-8 encoding to Latin 1 (ISO 8859-1):

-- convert a two-byte utf8 sequence to a Latin 1 character local function f2 (s) local c1, c2 = string.byte(s, 1, 2) return string.char(c1 * 64 + c2 - 12416) end local utf8 = lpeg.R("\0\127") + lpeg.R("\194\195") * lpeg.R("\128\191") / f2 local decode_pattern = lpeg.Cs(utf8^0) * -1

In this code, the definition of utf-8 is already restricted to the Latin 1 range (from 0 to 255). Any encoding outside this range (as well as any invalid encoding) will not match that pattern.

As the definition of `decode_pattern`

demands that
the pattern matches the whole input (because of the -1 at its end),
any invalid string will simply fail to match,
without any useful information about the problem.
We can improve this situation redefining `decode_pattern`

as follows:

local function er (_, i) error("invalid encoding at position " .. i) end local decode_pattern = lpeg.Cs(utf8^0) * (-1 + lpeg.P(er))

Now, if the pattern `utf8^0`

stops
before the end of the string,
an appropriate error function is called.

### UTF-8 and Unicode

We can extend the previous patterns to handle all Unicode code points. Of course, we cannot translate them to Latin 1 or any other one-byte encoding. Instead, our translation results in a array with the code points represented as numbers. The full code is here:

-- decode a two-byte utf8 sequence local function f2 (s) local c1, c2 = string.byte(s, 1, 2) return c1 * 64 + c2 - 12416 end -- decode a three-byte utf8 sequence local function f3 (s) local c1, c2, c3 = string.byte(s, 1, 3) return (c1 * 64 + c2) * 64 + c3 - 925824 end -- decode a four-byte utf8 sequence local function f4 (s) local c1, c2, c3, c4 = string.byte(s, 1, 4) return ((c1 * 64 + c2) * 64 + c3) * 64 + c4 - 63447168 end local cont = lpeg.R("\128\191") -- continuation byte local utf8 = lpeg.R("\0\127") / string.byte + lpeg.R("\194\223") * cont / f2 + lpeg.R("\224\239") * cont * cont / f3 + lpeg.R("\240\244") * cont * cont * cont / f4 local decode_pattern = lpeg.Ct(utf8^0) * -1

### Lua's Long Strings

A long string in Lua starts with the pattern `[=*[`

and ends at the first occurrence of `]=*]`

with
exactly the same number of equal signs.
If the opening brackets are followed by a newline,
this newline is discharged
(that is, it is not part of the string).

To match a long string in Lua, the pattern must capture the first repetition of equal signs and then, whenever it finds a candidate for closing the string, check whether it has the same number of equal signs.

open = "[" * lpeg.C(lpeg.P"="^0) * "[" * lpeg.P"\n"^-1 close = "]" * lpeg.C(lpeg.P"="^0) * "]" closeeq = lpeg.Cmt(close * lpeg.Cb(2), function (s, i, a, b) return a == b end) string = open * m.C((lpeg.P(1) - closeeq)^0) * close / function (o, s) return s end

The `open`

pattern matches `[=*[`

,
capturing the repetitions of equal signs for later use;
it also discharges an optional newline, if present.
The `close`

pattern matches `]=*]`

.
The `closeeq`

pattern first matches `close`

;
then it uses a back capture to recover the capture made
by the previous `open`

(the immediate previous capture is the one just made by `close`

,
so it must get the second previous capture);
finally it uses a match-time capture to check
whether both captures are equal.
The `string`

pattern starts with an `open`

,
then it goes as far as possible until matching `closeeq`

,
and then matches the final `close`

.
The final function capture simply consumes
the captures made by `open`

and `close`

and returns only the middle capture,
which is the string contents.

### Arithmetic Expressions

This example is a complete parser and evaluator for simple arithmetic expressions. We write it in two styles. The first approach first builds a syntax tree and then traverses this tree to compute the expression value:

-- Lexical Elements local Space = lpeg.S(" \n\t")^0 local Number = lpeg.C(lpeg.P"-"^-1 * lpeg.R("09")^1) * Space local FactorOp = lpeg.C(lpeg.S("+-")) * Space local TermOp = lpeg.C(lpeg.S("*/")) * Space local Open = "(" * Space local Close = ")" * Space -- Grammar local Exp, Term, Factor = lpeg.V"Exp", lpeg.V"Term", lpeg.V"Factor" G = lpeg.P{ Exp, Exp = lpeg.Ct(Factor * (FactorOp * Factor)^0); Factor = lpeg.Ct(Term * (TermOp * Term)^0); Term = Number + Open * Exp * Close; } G = Space * G * -1 -- Evaluator function eval (x) if type(x) == "string" then return tonumber(x) else local op1 = eval(x[1]) for i = 2, #x, 2 do local op = x[i] local op2 = eval(x[i + 1]) if (op == "+") then op1 = op1 + op2 elseif (op == "-") then op1 = op1 - op2 elseif (op == "*") then op1 = op1 * op2 elseif (op == "/") then op1 = op1 / op2 end end return op1 end end -- Parser/Evaluator function evalExp (s) local t = lpeg.match(G, s) if not t then error("syntax error", 2) end return eval(t) end -- small example print(evalExp"3 + 5*9 / (1+1) - 12")

The second style computes the expression value on the fly, without building the syntax tree. The following grammar takes this approach. (It assumes the same lexical elements as before.)

-- Auxiliary function function eval (v1, op, v2) if (op == "+") then return v1 + v2 elseif (op == "-") then return v1 - v2 elseif (op == "*") then return v1 * v2 elseif (op == "/") then return v1 / v2 end end -- Grammar local V = lpeg.V G = lpeg.P{ "Exp", Exp = lpeg.Ca(V"Factor" * (FactorOp * V"Factor" / eval)^0); Factor = lpeg.Ca(V"Term" * (TermOp * V"Term" / eval)^0); Term = Number / tonumber + Open * V"Exp" * Close; } -- small example print(lpeg.match(G, "3 + 5*9 / (1+1) - 12"))

Note the use of the accumulator capture.
To compute the value of an expression,
the accumulator starts with the value of the first factor,
and then applies `eval`

over
the accumulator, the operator,
and the new factor for each repetition.

## Download

LPeg source code.

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