real-regex 2026.7.24

REAL — linear-time (ReDoS-safe) regex engine with an re-compatible API

real-regex

A drop-in replacement for Python's re, backed by a linear-time, ReDoS-safe C++20 engine — with bounded lookarounds that the other linear-time engines (RE2, Rust) don't have.

pip install real-regex

Drop-in for re

Import it as re and use the API you already know — compile, search, match, fullmatch, findall, finditer, sub, subn, split, and Match.group / groups / groupdict / span / start / end:

import real as re

m = re.match(r"(\w+)@(\w+)\.(\w+)", "[e-mail]")
m.group(1), m.group(2), m.group(3)          # ('user', 'example', 'com')

re.findall(r"\d+", "a1 b22 c333")            # ['1', '22', '333']
re.sub(r"\s+", "_", "a  b   c")              # 'a_b_c'

Flags work the same: re.I, re.M, re.S, re.X, re.A. Unicode \w \d \s \b and IGNORECASE case-folding follow re in text mode.

Why replace re

• No ReDoS. re (like every backtracking engine) blows up on patterns like (a+)+b against a hostile input — seconds, then minutes. real-regex is a Thompson NFA simulation: linear time, always, no catastrophic backtracking, ever.
• Bounded lookarounds, still linear. (?=…) (?!…) (?<=…) (?<!…) — including variable-width lookbehind — match without backtracking. RE2 and Rust drop lookarounds to stay safe; real-regex keeps them.
• Faster than re on most work — geometric mean 2.06× across a broad corpus (CI [1.37, 3.12]). One-pass extraction and a lazy DFA make sparse findall 9.6×, multiline anchors 30×, and capture-dense group extraction (emails: 1.07×, up from a 0.32× loss before the one-pass work) beat re; a couple of high-volume split/tiny-anchored cases stay slower (CPython's C engine wins there). It is not the fastest engine at raw throughput — it is the safe one that is also quick.

Not re, on purpose

Strict by default — every accepted pattern is guaranteed linear. The unsupported constructs (backreferences, conditionals, binary Unicode properties like \p{Alphabetic}) raise real.error rather than silently falling back to a backtracking engine, so a compiled pattern is a ReDoS-safety guarantee, not a maybe. (\p{L} / \p{sc=Greek} — General_Category and Script — are supported, a superset of stdlib re.) Opt into delegating those to the standard library re per call with real.compile(pat, fallback=True) (or module-wide with real.fallback = True) — it may accept them, at the cost of the linear-time guarantee for that pattern; Pattern.engine ("real" or "re") always tells you which backend you got. A few semantics differ deliberately (e.g. a nullable loop's final empty capture). Full list: COMPATIBILITY.md.

The wheel also ships the header-only C++ library — real.get_include() returns its path, so the same engine is available to C++ via #include <real/real.hpp>. Source, benchmarks and the C++ API: https://github.com/RECHE23/real-regex.