Craft CMS defines a literal filter, which conflicts with the new literal operator introduced in Twig 3.21 via #4543.

For example, the following template will trigger the error: Unexpected token "operator" of value "literal" ("name" expected).

{{ 'foo' | literal }}

Lexer::getOperatorRegex() is defensive against filters being confused for operators here:

However the lookbehind doesn’t account for filters that have spaces around them, which is pretty commonplace.

Suggested fix

Changing that lookbehind code to the following fixes the problem:

$r = '(?<![\.\|]\s|.[\.\|])'.$r;

(It’s not possible to have variable-length lookbehinds, so I believe this is the only way to do it, with that . and all, rather than just (?<![\.\|]\s?).)