Also 'begin' is vague - it could mean start of word, here it means start of line. Make it 'start of line'
I like the general approach, although the 2010-era BDD fake natural language is a turn off.
A really good shorthand for "start of line" is "^"
;o)
A really good shorthand for "yes, however it's not the most discoverable thing in the world" is "& ^ ( ^ ^ % & ^"
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.
Whatever suits the user I suppose.
The word "either" implies only two choices, making your opening example confusing when the first "either of" was really picking from three possibilities.