How to define simple character from a string as a regex that has no reserved words in perl

I have a case like this: Apart from the reserved words with special meanings, any simple character in a regular expression should be equivalent to itself enclosed by the brackets, that is, “a” is equivalent to “[a]” if a is not a reserved word. In MR3, rf is constructed by replacing some simple characters in rs with their bracketed formats. For example, if rs contains a sub-expression “abc”, then we have “[a][b][c]” instead in rf.

This is an example of the regular expression of the program : %a-c]-[0-9]@t*@*?% How can I define the simple character of that regex that has no reserved words in perl?