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Regex

Regex syntax

Kakoune regex syntax is based on ECMAScript syntax, as defined by the ECMA-262 standard (see :doc regex compatibility).

Kakoune’s regex always runs 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 that \$ 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 newline 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 ^ 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 newline 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 uppercase basic letters. [A-Z0-9] will match all uppercase 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 digits 0-9.

  • \w which matches word characters A-Z, a-z, 0-9 and underscore (ignoring the extra_word_chars option).

  • \s which matches all Unicode whitespace characters.

  • \h which matches whitespace except Vertical Tab and line-breaks.

Using an upper case letter instead of a lower case one will negate the character class. For example, \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 newlines, by default. (see :doc regex modifiers on how to change it)

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 time.

  • * 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 they will prevent the regex from matching if not fulfilled.

  • ^ matches at the start of a line; that is, just after a newline character, or at the subject’s beginning (unless it is specified that the subject’s beginning is not a start of line).

  • $ matches at the end of a line; that is, just before a newline, or at the subject end (unless it is specified that the subject’s end is not an end of line).

  • \b matches at a word boundary; which is to say that between the previous character and the current character, one is a word character, and the other is not.

  • \B matches at a non-word boundary; meaning, when both the previous character and the current character are word characters, or both are not.

  • \A matches at the subject string’s beginning.

  • \z matches at the subject string’s end.

  • \K matches anything, and resets the start position of 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 a sequence of literals, 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) starts case-insensitive matching.

  • (?I) starts case-sensitive matching (default).

  • (?s) allows . to match newlines (default).

  • (?S) prevents . from matching newlines.

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

Kakoune’s syntax tries to follow the ECMAScript regex syntax, as defined by https://www.ecma-international.org/ecma-262/8.0/; some divergence exists for ease of use, or performance reasons:

  • 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 being 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.