Possible Unicode problem with string function search

Whilst writing a string replace function for Mathcad Express, I found what seems to be a problem with the string function search.   I wanted to check a string for the existence of the Unicode characters for 11 (↊) and 12 (↋) in the duodecimal system ... and it found them everywhere even when they weren't there at all.

Has this been noticed before?  Have I made some kind of error or unwitting assumption?

Cheers,

Stuart

(The simple recursive algorithm shown above is O(n) and only good for strings shorter than about 4000 characters (ie, under Mathcad's recursion depth limit.   A slightly more complicated algorithm would use a binary-division algorithm to reduce the recursion stack to O(log n), which will handle much longer strings)