Add support for custom character classes [...]

[SicroAtGit]

[x], while x can be several mixes of the following:

• Symbol:
  • Single character
  • Escape sequence
• Range:
  • a-c for the characters a, b and c

To negate the user-defined character class, the ^ character must follow immediately after the opening square bracket. If the ^ occurs in other positions, it is a normal character, without the special meaning.

The metacharacter . (dot) is a dot character inside the square brackets, i.e. without the special meaning.

Nesting of character classes is not allowed.

For ranges, the range end symbol must have a larger character code than the range start symbol, i.e. [a-z] instead of [z-a].