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+)", "user@example.com")
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, non-binary Unicode properties like \p{Bidi_Class=L}) 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} / \p{scx=Grek} / \p{Alphabetic} — General_Category, Script, Script_Extensions and the standard binary properties — 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.

Project links

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  • Released:
Newer release available (2026.8.17)

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Credits

Author: René Chenard

GitHub Statistics

Data verified by PyPI on Jul 16, 2026
The GitHub source repository was provided by the project maintainers and verified by PyPI at the time of upload. Stars, forks, and open issues/PRs are derived from that repository and have not been independently verified.

License expression

MIT
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Requires

Python >=3.10

Tags

regex regular-expression redos linear-time re

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