Processing of the pattern argument in yangtools fails for some valid XML schema regular expressions as required by RFC 7950.

For example, yangtools failed to process the following pattern:

 pattern '[\i-[:]][\c-[:]]*';

It appears that yangtools do not recognize the following multi-character escape sequences:

• \c
• \C
• \i
• \I

And also it does not interpret character class subtraction correctly and pattern matching does not work.

For example, the following pattern should not match 'c' but it does:

pattern '[a-z-[c]]'

Proposal solution:

Implement logic in the RegexUtils class to translate XSD patterns into Java-compatible regex. This can be achieved similarly to the method shown in this code:

https://github.com/apache/netbeans/blob/e8fb660e9fe47f14991e46a22ffcb38bf37bab75/ide/schema2beans/src/org/netbeans/modules/schema2beansdev/SchemaRep.java#L1624