The word "either" implies only two choices, making your opening example confusing when the first "either of" was really picking from three possibilities.
Is it less discoverable than the proposed alternative though?
In both cases one looks up the documentation, in one I find that search for the line start requires a regex with "^" and in the other I find something like "begin with" of Simple Regex Language (SRL). I still need to read (or test) to find what "begin with" means and I still couldn't guess it - why not "start with", "open with", "first character", or a myriad of other possible options.
Since code is read more than it is written, how well do you think a colleague without previous knowledge of regexs could understand '^' vs 'start of line'?
The word "either" implies only two choices, making your opening example confusing when the first "either of" was really picking from three possibilities.