Calling match with pos breaks us below 100ms on the benchmark.

Requires a patch to base/regex.jl to take the 'a' option for
anchoring matches. Otherwise you can't match from starting exactly
at a position.
By not copying the string every time you want to match in the middle
saves tons of memory and bumps benchmark as low as 70ms.

May be even faster to not copy any of it and make a better beginswith()