Match this pattern against the start of string.
If start is provided, it must be an integer in the range 0 ..
string.length. In that case, this patten is tested against the
string at the start position. That is, a Match is returned if the
pattern can match a part of the string starting from position start.
Returns null if the pattern doesn't match.
Source
Match matchAsPrefix(String string, [int start = 0]);