`DEFAULT_EXP` is by far the most common regexp being anchored.
By special casing it, we can re-use the existing `DEFAULT_EXP_ANCHORED`
regexp, and avoid keeping lots of copies of it in memory. Every time
we hit, we also avoid having to compile the regexp, so it's faster.

We also replace the `Regexp.new(/\A#{sym}\Z/)` pattern by just
`/\A#{sym}\Z/`, as it was compiling and instantiating an extra
regexp for no good reason.