kakoune/doc/pages/regex.asciidoc

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= Regex
== Regex Syntax
Kakoune regex syntax is based on the ECMAScript syntax, as defined by the
ECMA-262 standard (see <<Compatibility>>).
Kakoune's regex always run on Unicode codepoint sequences, not on bytes.
== Literals
Every character except the syntax characters `\^$.*+?[]{}|().` match
themselves. Syntax characters can be escaped with a backslash so `\$`
will match a literal `$` and `\\` will match a literal `\`.
Some literals are available as escape sequences:
* `\f` matches the form feed character.
* `\n` matches the line feed character.
* `\r` matches the carriage return character.
* `\t` matches the tabulation character.
* `\v` matches the vertical tabulation character.
* `\0` matches the null character.
* `\cX` matches the control-X character (X can be in `[A-Za-z]`).
* `\xXX` matches the character whose codepoint is XX (in hexadecimal).
* `\uXXXXXX` matches the character whose codepoint is XXXXXX (in hexadecimal).
== Character classes
The `[` character introduces a character class, matching one character
from a set of characters.
A character class contains a list of literals, character ranges,
and character class escapes surrounded by `[` and `]`.
If the first character inside a character class is `^`, then the character
class is negated, meaning that it matches every character not specified
in the character class.
Literals match themselves, including syntax characters, so `^`
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does not need to be escaped in a character class. `[\*+]` matches both
the `\*` character and the `+` character. Literal escape sequences are
supported, so `[\n\r]` matches both the line feed and carriage return
characters.
The `]` character needs to be escaped for it to match a literal `]`
instead of closing the character class.
Character ranges are written as `<start character>-<end character>`, so
`[A-Z]` matches all upper case basic letters. `[A-Z0-9]` will match all
upper cases basic letters and all basic digits.
The `-` characters in a character class that are not specifying a
range are treated as literal `-`, so `[A-Z-+]` matches all upper case
characters, the `-` character, and the `+` character.
Supported character class escapes are:
* `\d` which matches all digits.
* `\w` which matches all word characters.
* `\s` which matches all whitespace characters.
* `\h` which matches all horizontal whitespace characters.
Using an upper case letter instead of a lower case one will negate
the character class, meaning for example that `\D` will match every
non-digit character.
Character class escapes can be used outside of a character class, `\d`
is equivalent to `[\d]`.
== Any character
`.` matches any character, including new lines.
== Groups
Regex atoms can be grouped using `(` and `)` or `(?:` and `)`. If `(` is
used, the group will be a capturing group, which means the positions from
the subject strings that matched between `(` and `)` will be recorded.
Capture groups are numbered starting at 1. They are numbered in the
order of appearance of their `(` in the regex. A special capture group
0 is for the whole sequence that matched.
* `(?:` introduces a non capturing group, which will not record the
matching positions.
* `(?<name>` introduces a named capturing group, which, in addition to
being referred by number, can be, in certain contexts, referred by the
given name.
== Alternations
The `|` character introduces an alternation, which will either match
its left-hand side, or its right-hand side (preferring the left-hand side)
For example, `foo|bar` matches either `foo` or `bar`, `foo(bar|baz|qux)`
matches `foo` followed by either `bar`, `baz` or `qux`.
== Quantifier
Literals, Character classes, Any characters and groups can be followed
by a quantifier, which specifies the number of times they can match.
* `?` matches zero or one times.
* `*` matches zero or more times.
* `+` matches one or more times.
* `{n}` matches exactly n times.
* `{n,}` matches n or more times.
* `{n,m}` matches n to m times.
* `{,m}` matches zero to m times.
By default, quantifiers are *greedy*, which means they will prefer to
match more characters if possible. Suffixing a quantifier with `?` will
make it non-greedy, meaning it will prefer to match as few characters
as possible.
== Zero width assertions
Assertions do not consume any character, but will prevent the regex
from matching if they are not fulfilled.
* `^` matches at the start of a line, that is just after a new line
character, or at the subject begin (except if specified that the
subject begin is not a start of line).
* `$` matches at the end of a line, that is just before a new line, or
at the subject end (except if specified that the subject's end
is not an end of line).
* `\b` matches at a word boundary, when one of the previous character
and current character is a word character, and the other is not.
* `\B` matches at a non word boundary, when both the previous character
and the current character are word, or are not.
* `\A` matches at the subject string begin.
* `\z` matches at the subject string end.
* `\K` matches anything, and resets the start position of the capture
group 0 to the current position.
More complex assertions can be expressed with lookarounds:
* `(?=...)` is a lookahead, it will match if its content matches the text
following the current position
* `(?!...)` is a negative lookahead, it will match if its content does
not match the text following the current position
* `(?<=...)` is a lookbehind, it will match if its content matches
the text preceding the current position
* `(?<!...)` is a negative lookbehind, it will match if its content does
not match the text preceding the current position
For performance reasons lookaround contents must be sequence of literals,
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character classes or any-character (`.`); Quantifiers are not supported.
For example, `(?<!bar)(?=foo).` will match any character which is not
preceded by `bar` and where `foo` matches from the current position
(which means the character has to be an `f`).
== Modifiers
Some modifiers can control the matching behavior of the atoms following
them:
* `(?i)` enables case-insensitive matching
* `(?I)` disables case-insensitive matching (default)
* `(?s)` enables dot-matches-newline (default)
* `(?S)` disables dot-matches-newline
== Quoting
`\Q` will start a quoted sequence, where every character is treated as
a literal. That quoted sequence will continue until either the end of
the regex, or the appearance of `\E`.
For example `.\Q.^$\E$` will match any character followed by the literal
string `.^$` followed by an end of line.
== Compatibility
The syntax tries to follow the ECMAScript regex syntax as defined by
https://www.ecma-international.org/ecma-262/8.0/ some divergences
exists for ease of use or performance reasons:
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* lookarounds are not arbitrary, but lookbehind is supported.
* `\K`, `\Q..\E`, `\A`, `\h` and `\z` are added.
* Stricter handling of escaping, as we introduce additional
escapes, identity escapes like `\X` with X a non-special character
are not accepted, to avoid confusions between `\h` meaning literal
`h` in ECMAScript, and horizontal blank in Kakoune.
* `\uXXXXXX` uses 6 digits to cover all of unicode, instead of relying
on ECMAScript UTF-16 surrogate pairs with 4 digits.