Anchors

Anchors match positions in the input rather than characters. They are zero-width assertions: they check where something is allowed to match, but they never consume input themselves.

Anchors are essential when you want to control where a pattern may appear, for example at the start of the input, at line boundaries, or between word and non-word characters.

\A

Matches only at the start of the input.

This anchor is independent of multiline mode and always refers to the absolute beginning of the input. It is equivalent to ^ when multiline mode is disabled, but remains unaffected when multiline mode is enabled.

Use this anchor when you need a strict guarantee that matching begins at the very start.

\b

Matches a word boundary, that is, the position between a word character and a non-word character.

What counts as a word character depends on the active ASCII or Unicode mode (see \w). The start and end of the input are treated as non-word positions, which means \b can also match at the very beginning or end of the input if the adjacent character is a word character.

This anchor is commonly used to ensure that identifiers, keywords, or tokens are matched as whole words rather than as substrings.

\B

Matches a non-word boundary, meaning a position between two word characters or between two non-word characters.

The same ASCII or Unicode rules apply as for \b. This anchor is essentially the logical opposite of \b and is useful when you explicitly want to avoid word boundaries.

\Z

Matches only at the end of the input.

This anchor does not match before a trailing newline. It always refers to the true end of the input and is equivalent to \z and $ when multiline mode is disabled.

Use this anchor when matching must stop exactly at the input boundary.

^

Matches the start of the input, or the start of a line when multiline mode is enabled.

You can enable multiline mode either programmatically via Flag::Multiline or inline using (?m). In multiline mode, ^ matches immediately after a line feed (\n) or at the start of the input.

In single-line mode, ^ is equivalent to \A.

$

Matches the end of the input, or the end of a line when multiline mode is enabled.

Multiline mode can be enabled via Flag::Multiline or inline using (?m). In multiline mode, $ matches immediately before a line feed (\n) or at the end of the input.

In single-line mode, this anchor is equivalent to \Z.

Legacy and Compatibility Anchors

\z

Matches only at the end of the input (same behavior as \Z).

This anchor exists solely for compatibility with other regular expression engines. It can be disabled using Feature::AnchorLowercaseZ.

For new patterns, prefer \Z for clarity and consistency.

Notes and Compatibility

• Anchors such as \b and \B are not allowed inside character classes. If you need conditional behavior, use grouping instead, for example (?:[…]|\b).

• Unlike PCRE, \Z and $ in single-line mode do not match before a trailing newline. They only match at the true end of the input.

• \b is always interpreted as a word-boundary anchor. It is not an alias for the backspace character inside character classes. If you need a literal backspace, use \x08.