The re Module
The re module
(provided by file re.lua in the distribution)
supports a somewhat conventional regex syntax
for pattern usage within LPeg.
The next table summarizes re's syntax.
A p represents an arbitrary pattern;
num represents a number ([0-9]+);
name represents an identifier
([a-zA-Z][a-zA-Z0-9_]*).
Constructions are listed in order of decreasing precedence.
| Syntax | Description |
( p ) | grouping |
& p | and predicate |
! p | not predicate |
p1 p2 | concatenation |
p1 / p2 | ordered choice |
p ? | optional match |
p * | zero or more repetitions |
p + | one or more repetitions |
p^num |
exactly num repetitions |
p^+num |
at least num repetitions |
p^-num |
at most num repetitions |
(name <- p)+ | grammar |
'string' | literal string |
"string" | literal string |
[class] | character class |
. | any character |
%name |
pattern defs[name] or a pre-defined pattern |
name | non terminal |
<name> | non terminal |
{} | position capture |
{ p } | simple capture |
{: p :} | anonymous group capture |
{:name: p :} | named group capture |
{~ p ~} | substitution capture |
{| p |} | table capture |
=name | back reference |
p -> 'string' | string capture |
p -> "string" | string capture |
p -> num | numbered capture |
p -> name | function/query/string capture
equivalent to p / defs[name] |
p => name | match-time capture
equivalent to lpeg.Cmt(p, defs[name]) |
p ~> name | fold capture (deprecated) |
p >> name | accumulator capture
equivalent to (p % defs[name]) |
Any space appearing in a syntax description can be
replaced by zero or more space characters and Lua-style short comments
(-- until end of line).
Character classes define sets of characters.
An initial ^ complements the resulting set.
A range x-y includes in the set
all characters with codes between the codes of x and y.
A pre-defined class %name includes all
characters of that class.
A simple character includes itself in the set.
The only special characters inside a class are ^
(special only if it is the first character);
]
(can be included in the set as the first character,
after the optional ^);
% (special only if followed by a letter);
and -
(can be included in the set as the first or the last character).
Currently the pre-defined classes are similar to those from the
Lua's string library
(%a for letters,
%A for non letters, etc.).
There is also a class %nl
containing only the newline character,
which is particularly handy for grammars written inside long strings,
as long strings do not interpret escape sequences like \n.
Functions
re.compile (string, [, defs])
Compiles the given string and
returns an equivalent LPeg pattern.
The given string may define either an expression or a grammar.
The optional defs table provides extra Lua values
to be used by the pattern.
re.find (subject, pattern [, init])
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.
An optional numeric argument init makes the search
starts at that position in the subject string.
As usual in Lua libraries,
a negative value counts from the end.
re.gsub (subject, pattern, replacement)
Does a global substitution,
replacing all occurrences of pattern
in the given subject by replacement.
re.match (subject, pattern)
Matches the given pattern against the given subject, returning all captures.
re.updatelocale ()
Updates the pre-defined character classes to the current locale.
Some Examples
A complete simple program
The next code shows a simple complete Lua program using
the re module:
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
Balanced parentheses
The following call will produce the same pattern produced by the Lua expression in the balanced parentheses example:
b = re.compile[[ balanced <- "(" ([^()] / balanced)* ")" ]]
String reversal
The next example reverses a string:
rev = re.compile[[ R <- (!.) -> '' / ({.} R) -> '%2%1']]
print(rev:match"0123456789") --> 9876543210
CSV decoder
The next example replicates the CSV decoder:
record = re.compile[[
record <- {| field (',' field)* |} (%nl / !.)
field <- escaped / nonescaped
nonescaped <- { [^,"%nl]* }
escaped <- '"' {~ ([^"] / '""' -> '"')* ~} '"'
]]
Lua's long strings
The next example matches Lua long strings:
c = re.compile([[
longstring <- ('[' {:eq: '='* :} '[' close)
close <- ']' =eq ']' / . close
]])
print(c:match'[==[]]===]]]]==]===[]') --> 17
Abstract Syntax Trees
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 = re.compile[[
listname <- (name s)*
name <- [a-z][a-z]*
s <- %s*
]]
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:
c = re.compile[[
listname <- {| (name s)* |}
name <- {| {[a-z][a-z]*} |}
s <- %s*
]]
Now, a match against "hi hello bye"
results in the table
{{"hi"}, {"hello"}, {"bye"}}.
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 tag field telling what non terminal
that table represents.
We can add such a tag using
named group captures:
x = re.compile[[
listname <- {| {:tag: '' -> 'list':} (name s)* |}
name <- {| {:tag: '' -> 'id':} {[a-z][a-z]*} |}
s <- ' '*
]]
With these group captures,
a match against "hi hello bye"
results in the following table:
{tag="list",
{tag="id", "hi"},
{tag="id", "hello"},
{tag="id", "bye"}
}
Indented blocks
This example breaks indented blocks into tables, respecting the indentation:
p = re.compile[[
block <- {| {:ident:' '*:} line
((=ident !' ' line) / &(=ident ' ') block)* |}
line <- {[^%nl]*} %nl
]]
As an example, consider the following text:
t = p:match[[
first line
subline 1
subline 2
second line
third line
subline 3.1
subline 3.1.1
subline 3.2
]]
The resulting table t will be like this:
{'first line'; {'subline 1'; 'subline 2'; ident = ' '};
'second line';
'third line'; { 'subline 3.1'; {'subline 3.1.1'; ident = ' '};
'subline 3.2'; ident = ' '};
ident = ''}
Macro expander
This example implements a simple macro expander. Macros must be defined as part of the pattern, following some simple rules:
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)
A text is a sequence of items,
wherein we apply a substitution capture to expand any macros.
An item is either a macro,
any character different from parentheses,
or a parenthesized expression.
A macro argument (arg) 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.
args 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 %n replaced by the n-th argument.
Patterns
This example shows the complete syntax
of patterns accepted by re.
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
License
This module is part of the LPeg package and shares its license.