A parameter name missing leading and/or trailing _ is accepted, probably erroneously. For example, _modifiers: a Modifiers, (as currently in proposal-regexp-modifiers spec) renders like "_modifiers (a Modifiers)" with the literal _ visible and "modifiers" not wrapped in a <var> element (as currently in proposal-regexp-modifiers HTML).

The responsible code appears to be at

/^[A-Za-z0-9_] */i should probably be replaced with /^_[A-Za-z0-9] _ */i, or maybe /^(?:_[A-Za-z0-9] _|[A-Za-z0-9] ) */i if we want to support parameter names not wrapped in _ (which I'd personally be against). And incidentally, I also note that the i flag is redundant with A-Za-z.