Help with custom Actions to replace colon

Is there a way to configure an Action that would do the following:

  1. When a colon ( : ) appears immediately after anything other than a number, replace it with a period ( . )

  2. When the phrase " In A Minor" -- or any similar key designation -- appears, delete it

Yes, 1) is possible with a regular expression.
However, you need to specify more precisely what you mean by 'other than a number' and exactly where you want it.
For example do you want to replace in the field TITLE
"This is : colon"
becomes
"This is . colon"
Please show us some real examples.

For 2: This would only be possible if you define what you mean by 'or any similar key designation'. A regular expression cannot 'guess' what you mean. You need to define exactly for example
" in A Minor" -> delete
" in B Major" -> Delete
" in A Minority " - leave untouched
Please show us some real examples.

You could try as
Format string: $regexp(%title%,(\D+):,$1.)

For those interested:
\D+ means "any character that's not a digit, repeated at least once"
while
\d+ means "a digit, repeated at least once"

(\D+): captures one or more non-digit characters followed by a colon.
In the replacement string, $1 refers to the text captured by the first capture group, so $1. replaces each match with the captured text followed by a period.