Symfony version(s) affected: 3.4.11

Description
when parsing yaml files the method Yaml\Parser::isBlockScalarHeader is oftentimes on the hotpath, because doing a lot of preg_match calls.

see

would it make sense to do a cheaper fast-fail check before actually invoking the regex-engine?

I do not have yet a proper feeling of what this method is trying to check and why, so my suggestion might be rubbish.