This looks like a fairly fundamental bug in Str module..
(so probably I'm missing something ..) 

This program:

let m = Str.regexp "a";;
let b = Str.string_match m "aa" 0;;

print_endline (if b then "YES" else "NO");;

prints YES. Clearly the string "aa" is NOT matched by
the regexp "a".

This *would be* the correct result for the

Str.string_partial_match

function which matches prefixes. However Str.string_match
should match the whole string. [In bugtracker]

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