For Fuzzy Matching, it is impossible to specify a test character set for both insertion and substitution at the same time.

[Ptolemy2002]

A pattern such as this compiles:
(?:hello){s<=2,d<1,e<=3,i<=2:\d}

but the same pattern with a substitution character set added does not:
(?:hello){d<1,e<=3,i<=2:[\d],s<=2:[a-zA-Z]}

The error message is:

expected } at position 28`

The computer seems to expect a closing brace immediately after the specification of a character set, which means that you can only specify one character set, and it has to be at the end of the quantifier. I'm unsure if this is a bug or purposeful, but I can't see why the process should be restricted to one or the other.

[mrabarnett]

The additional test was requested in issue #338. It applies to both the insertions and the substitutions, so in (?:hello){s<=2,d<1,e<=3,i<=2:\d} the \d applies to both the s and the i. (Changing that would break existing code.) You're the first person who has requested the ability to have separate tests for insertions and substitutions.

[Ptolemy2002]

It applies to both the insertions and the substitutions

I didn't know this. Reading the documentation, I misunderstood the text "You can add a test to perform on a character that's substituted or inserted." Maybe that should be updated to be slightly more clear?