Boundaries

Boundaries (word boundaries and anchors) are assertions that match if the current position has a certain property.

Syntax #

let Boundary =
    | '^'
    | '$'
    | '%'
    | '<'
    | '>';

Example #

^ $           # match empty string
% 'foo' %     # match 'foo' surrounded by word boundaries
!% 'foo' !%   # match 'foo' not surrounded by word boundaries
< 'foo' >     # match 'foo' as a whole word

Support #

Anchors (^ and $) are supported in all flavors. Word boundaries (%, <, and >) are not supported in JavaScript unless Unicode is disabled.

Support for boundaries is gated by the boundaries feature. Specify features with the --allowed-features option.

Behavior #

All boundaries are assertions – they match between two characters. They do not contain any text, and repeating them has no effect.

Anchors #

^ and $ are anchors. They match at the start and end of the string, respectively. Regex engines usually have a way to change their behavior to match at the start and end of the line instead.

They have the built-in Start and End variables as aliases.

Word boundaries #

% is a word boundary, which matches either at the start or at the end of a word. < only matches at the start of a word, > only at the end. Surround a word with % % or with < > to make sure it doesn’t match a substring of a word, e.g. test in the word detest.

A word boundary is a position next to a “word character” (matching [word]), but only on one side. A word character is a character in one of the following Unicode general categories:

• Alphabetic
• Mark
• Decimal_Number
• Connector_Punctuation
• Join_Control

In the ASCII subset of Unicode, this would be the letters a-z and A-Z, the digits 0-9, and the underscore _.

The % word boundary is the only boundary that can be negated. !% matches a position that is not a word boundary, which means that it must be surrounded by either 0 or 2 word characters.

Relation to lookaround #

Every boundary can be expressed in terms of lookaround assertions:

Compilation #

Anchors are compiled verbatim to ^ and $. Word boundaries are compiled to \b, or \B when negated.

< and > are compiled to

• [[:<:]] and [[:>:]] when targeting PCRE
• \< and \> when targeting Rust
• (?<!\w)(?=\w) and (?<=\w)(?!\w) when targeting any other flavor

Issues #

In JavaScript, word boundaries are never Unicode aware, so they are only allowed when Unicode is explicitly disabled.

In other flavors, word boundaries are always Unicode aware, even when Unicode has been disabled.

History #

• Added < and > in Pomsky 0.11
• Forbidden % in JavaScript unless Unicode is disabled in Pomsky 0.10
• Removed deprecated <% and %> syntax in Pomsky 0.7
• Added ^ and $ in Pomsky 0.6
• Added Start and End variables in Pomsky 0.4.2
• Initial implementation in Pomsky 0.1
  • Using old syntax <% and %> instead of ^ and $