Some special characters are allowed only if they are preceded by an escape character

Question

I want to construct a regular expression (in the style of lex, with a more OCaml-like syntax) for a class of strings, where 4 characters [, ], #, ' are allowed only if they are preceded by an escape character '.

Here are some valid examples:

• '#Data, abc'#Headers, abc'#Totals'[efg, 123'#Totals']efg, abc, 123

Here are some non-valid examples:

• #Data, abc#Headers, abc#Totals[efg,123#Totals]efg, '#Totals[efg

Hope the definition is clear. First, does anyone know how to construct such a regular expression? Second, does anyone know how to construct such a regular expression (in the style of lex, with a more OCaml-like syntax) that can be accepted by ocamllex?

Answer 1

You don't say the accepted strings look like other than with a few examples. Just for concreteness, let's say that lower-case letters and digits are allowed, and the 4 special characters are allowed only if preceded by '.

This, then, is described by the Kleene closure of a set of 36 one-character strings and 4 two-character strings.

Which looks like this:

 (['a' - 'z' '0' - '9'] | '\'' ['\'' '#' '[' ']'])*