Added support of regular expressions not supported directly by PCRE2.

Added support of regular expressions not supported directly by PCRE2.

The following patterns were fixed:
    `[]` - matches nothing, previously was rejected as invalid expression.
    `[^]` - matched any character, unlike `.` this syntax matches new
        line, previously was rejected as invalid expression.
    `++`, `*+`, `?+` - are rejected now, whereas in PCRE2 they are considered
        valid possessive quantifiers.