This is an engineering choice by RE2 to not support backtracking or lookahead in exchange for runtime guarantees. PCRE2, which is not much more complex, supports both of these, but doesn't give the same runtime guarantees as RE2. For someone who is operating on vast quantities of unknown, unsanitized data, eg. Google, RE2s limitations are a reasonable tradeoff to make to avoid DOS situations from malformed or malicious inputs.
