memchr 2.6.4

Provides extremely fast (uses SIMD on x86_64, aarch64 and wasm32) routines for 1, 2 or 3 byte search and single substring search.


This library provides heavily optimized routines for string search primitives.

Build status

Dual-licensed under MIT or the UNLICENSE.



  • The top-level module provides routines for searching for 1, 2 or 3 bytes in the forward or reverse direction. When searching for more than one byte, positions are considered a match if the byte at that position matches any of the bytes.
  • The memmem sub-module provides forward and reverse substring search routines.

In all such cases, routines operate on &[u8] without regard to encoding. This is exactly what you want when searching either UTF-8 or arbitrary bytes.

Compiling without the standard library

memchr links to the standard library by default, but you can disable the std feature if you want to use it in a #![no_std] crate:

memchr = { version = "2", default-features = false }

On x86_64 platforms, when the std feature is disabled, the SSE2 accelerated implementations will be used. When std is enabled, AVX2 accelerated implementations will be used if the CPU is determined to support it at runtime.

SIMD accelerated routines are also available on the wasm32 and aarch64 targets. The std feature is not required to use them.

When a SIMD version is not available, then this crate falls back to SWAR techniques.

Minimum Rust version policy

This crate's minimum supported rustc version is 1.61.0.

The current policy is that the minimum Rust version required to use this crate can be increased in minor version updates. For example, if crate 1.0 requires Rust 1.20.0, then crate 1.0.z for all values of z will also require Rust 1.20.0 or newer. However, crate 1.y for y > 0 may require a newer minimum version of Rust.

In general, this crate will be conservative with respect to the minimum supported version of Rust.

Testing strategy

Given the complexity of the code in this crate, along with the pervasive use of unsafe, this crate has an extensive testing strategy. It combines multiple approaches:

  • Hand-written tests.
  • Exhaustive-style testing meant to exercise all possible branching and offset calculations.
  • Property based testing through quickcheck.
  • Fuzz testing through cargo fuzz.
  • A huge suite of benchmarks that are also run as tests. Benchmarks always confirm that the expected result occurs.

Improvements to the testing infrastructure are very welcome.

Algorithms used

At time of writing, this crate's implementation of substring search actually has a few different algorithms to choose from depending on the situation.

  • For very small haystacks, Rabin-Karp is used to reduce latency. Rabin-Karp has very small overhead and can often complete before other searchers have even been constructed.
  • For small needles, a variant of the "Generic SIMD" algorithm is used. Instead of using the first and last bytes, a heuristic is used to select bytes based on a background distribution of byte frequencies.
  • In all other cases, Two-Way is used. If possible, a prefilter based on the "Generic SIMD" algorithm linked above is used to find candidates quickly. A dynamic heuristic is used to detect if the prefilter is ineffective, and if so, disables it.