Write a Regex Engine Subset That Can Describe Token Patterns

Implement a subset of a regex engine for matching token patterns.

Requirements:

• Support: alternation (|), concatenation, Kleene star (*), plus (+), optional (?)
• Support: character classes [a-z], escaped characters . \d \w
• Support: anchors ^ and $ (start/end of string)
• Implement NFA simulation (Thompson NFA)
• Test regex patterns for identifiers, numbers, strings
• Match function returns true/false if pattern matches input
• Handle complex patterns like [a-zA-Z_][a-zA-Z0-9_]*