Putting It All Together

You've built every layer:

What separates a toy from re2 / Go's regexp:

• DFA cache: convert subsets of NFA states into DFA states on the fly. Cache the transitions. After the first run over each input pattern, subsequent runs are constant work per character.
• Unicode: \d should match Devanagari digits, \w should match Greek letters. Real engines have Unicode tables.
• Encoding: UTF-8 byte-level matching is faster than per-codepoint. RE2 operates on bytes; results are reported as byte offsets.
• Compile-time validation: catch unbalanced parens, invalid backref numbers, character class boundary errors with good error messages.
• Lookarounds: (?=...) (positive lookahead) and (?!...) (negative) let you assert without consuming. Hard in pure NFA — most engines fall back to backtracking for these.

What you've built handles:

• Most useful regex patterns
• Linear time (no catastrophic backtracking)
• Captures
• Real-world readability (your code matches Thompson's 1968 paper)

You now understand what powers grep, sed, awk, vim, perl, and almost every other tool that needs to find patterns in text.