[vLLM structured outputs] Regex constraints rejected: "Regex uses unsupported feature for structured outputs ... lookarounds, backreferences, and unicode boundaries are not." / "Regex compilation tim…

Cause (Documented platform behavior): Structured output backends compile regexes to automata (Rust-style regex, except lm-format-enforcer uses Python re) and cannot express context-dependent constructs; compilation also has a timeout. Fix status: documented_behavior Other error fragments: - Regex does not have a anchored universal start state - Regex compilation timed out after {timeout}s. The pattern may be too complex or contain constructs that cause exponential state-space explosion (e.g. nested quantifiers). Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/vllm-project/vllm/e7900156e130c9880eb03b7c1f2df32820e7a2be/vllm/v1/structured_output/backend_outlines.py (official_docs, unknown, documented_behavior): Regex validation raises for unsupported constructs and for patterns without an anchored universal start state. - https://raw.githubusercontent.com/vllm-project/vllm/e7900156e130c9880eb03b7c1f2df32820e7a2be/vllm/v1/structured_output/utils.py (official_docs, unknown, documented_behavior): Regex compilation timeout error mentions exponential state-space explosion. - https://raw.githubusercontent.com/vllm-project/vllm/e7900156e130c9880eb03b7c1f2df32820e7a2be/docs/features/structured_outputs.md (official_docs, unknown, documented_behavior): Docs: regex syntax depends on backend; xgrammar/guidance/outlines use Rust-style regex, lm-format-enforcer uses Python re. Search phrasings: vllm regex uses unsupported feature for structured outputs; vllm guided_regex lookahead; vllm regex compilation timed out structured outputs Evidence basis (self-declared by the contributing chat client): public_source.