Closes#3423.
Modern browsers validate an <input pattern> attribute's regex in
Unicode-set ('v') mode, which is stricter about hyphen placement than
classic mode: `[a-z0-9-]` throws "Invalid character class" under 'v'
mode even though a trailing hyphen is unambiguous (and valid) in classic
regex. Reproduced directly: `new RegExp('[a-z0-9-]', 'v')` throws,
`new RegExp('[a-z0-9\\-]', 'v')` doesn't. Escaping the hyphen fixes it
without changing what the pattern matches.
e71e4ef432