RE2 is a C++ software library which implements a regular expression engine. It uses finite-state machines, in contrast to most other regular expression libraries. RE2 requires a minimum C++ version of C++17, and uses the Abseil library by Google.

RE2 was implemented by Google and Google uses RE2 for Google products. RE2 uses an "on-the-fly" deterministic finite-state automaton algorithm based on Ken Thompson's Plan 9 grep. It is designed to avoid ReDoS (regex denial of service) attacks.

RE2 performs comparably to Perl Compatible Regular Expressions (PCRE). For certain regular expression operators like | (the operator for alternation or logical disjunction) it is superior to PCRE. Unlike PCRE, which supports features such as lookarounds, backreferences and recursion, RE2 is only able to recognize regular languages due to its construction using the Thompson DFA algorithm. It is also slightly slower than PCRE for parenthetic capturing operations.

PCRE can use a large recursive stack with corresponding high memory usage and result in exponential runtime on certain patterns. In contrast, RE2 uses a fixed stack size and guarantees that its runtime increases linearly (not exponentially) with the size of the input. The maximum memory allocated with RE2 is configurable. This can make it more suitable for use in server applications, which require boundaries on memory usage and computational time.

RE2 is available to users of Google Docs and Google Sheets. Google Sheets supports RE2 except Unicode character class matching. RegexExtract does not use grouping.

Here is an example of using re2 against a potential ReDoS (regular expression denial of service) attack.

RE2 comes with a built-in Python wrapper, available on Python Package Index (PyPI) as google-re2.

The built-in regexp package in Go uses the same patterns and implementation as RE2, though it is written in Go. This is unsurprising, given Go's common staff from the Plan 9 team.