Simplify and fix IsRegex detection by looking for special chars used by regular expression syntax. Add unit test.

Implementation of IsRegex was buggy and did not pass most of test cases attached in unit test. 
It tried to analyze the string to check if it has valid regular expression syntax.

New implementation take the problem by simple approach: if string contain regex syntax chars it's potentially a regular expression.

For checking is regex syntax is valid there is a seperate function in API to do this task.