gecko-dev/third_party/rust/regex/PERFORMANCE.md

Your friendly guide to understanding the performance characteristics of this crate.

This guide assumes some familiarity with the public API of this crate, which can be found here: https://docs.rs/regex

Theory vs. Practice

One of the design goals of this crate is to provide worst case linear time behavior with respect to the text searched using finite state automata. This means that, in theory, the performance of this crate is much better than most regex implementations, which typically use backtracking which has worst case exponential time.

For example, try opening a Python interpreter and typing this:

>>> import re
>>> re.search('(a*)*c', 'a' * 30).span()

I'll wait.

At some point, you'll figure out that it won't terminate any time soon. ^C it.

The promise of this crate is that this pathological behavior can't happen.

With that said, just because we have protected ourselves against worst case exponential behavior doesn't mean we are immune from large constant factors or places where the current regex engine isn't quite optimal. This guide will detail those cases and provide guidance on how to avoid them, among other bits of general advice.

Thou Shalt Not Compile Regular Expressions In A Loop

Advice: Use lazy_static to amortize the cost of Regex compilation.

Don't do it unless you really don't mind paying for it. Compiling a regular expression in this crate is quite expensive. It is conceivable that it may get faster some day, but I wouldn't hold out hope for, say, an order of magnitude improvement. In particular, compilation can take any where from a few dozen microseconds to a few dozen milliseconds. Yes, milliseconds. Unicode character classes, in particular, have the largest impact on compilation performance. At the time of writing, for example, \pL{100} takes around 44ms to compile. This is because \pL corresponds to every letter in Unicode and compilation must turn it into a proper automaton that decodes a subset of UTF-8 which corresponds to those letters. Compilation also spends some cycles shrinking the size of the automaton.

This means that in order to realize efficient regex matching, one must amortize the cost of compilation. Trivially, if a call to is_match is inside a loop, then make sure your call to Regex::new is outside that loop.

In many programming languages, regular expressions can be conveniently defined and compiled in a global scope, and code can reach out and use them as if they were global static variables. In Rust, there is really no concept of life-before-main, and therefore, one cannot utter this:

static MY_REGEX: Regex = Regex::new("...").unwrap();

Unfortunately, this would seem to imply that one must pass Regex objects around to everywhere they are used, which can be especially painful depending on how your program is structured. Thankfully, the lazy_static crate provides an answer that works well:

#[macro_use] extern crate lazy_static;
extern crate regex;

use regex::Regex;

fn some_helper_function(text: &str) -> bool {
    lazy_static! {
        static ref MY_REGEX: Regex = Regex::new("...").unwrap();
    }
    MY_REGEX.is_match(text)
}

In other words, the lazy_static! macro enables us to define a Regex as if it were a global static value. What is actually happening under the covers is that the code inside the macro (i.e., Regex::new(...)) is run on first use of MY_REGEX via a Deref impl. The implementation is admittedly magical, but it's self contained and everything works exactly as you expect. In particular, MY_REGEX can be used from multiple threads without wrapping it in an Arc or a Mutex. On that note...

Using a regex from multiple threads

Advice: The performance impact from using a Regex from multiple threads is likely negligible. If necessary, clone the Regex so that each thread gets its own copy. Cloning a regex does not incur any additional memory overhead than what would be used by using a Regex from multiple threads simultaneously. Its only cost is ergonomics.

It is supported and encouraged to define your regexes using lazy_static! as if they were global static values, and then use them to search text from multiple threads simultaneously.

One might imagine that this is possible because a Regex represents a compiled program, so that any allocation or mutation is already done, and is therefore read-only. Unfortunately, this is not true. Each type of search strategy in this crate requires some kind of mutable scratch space to use during search. For example, when executing a DFA, its states are computed lazily and reused on subsequent searches. Those states go into that mutable scratch space.

The mutable scratch space is an implementation detail, and in general, its mutation should not be observable from users of this crate. Therefore, it uses interior mutability. This implies that Regex can either only be used from one thread, or it must do some sort of synchronization. Either choice is reasonable, but this crate chooses the latter, in particular because it is ergonomic and makes use with lazy_static! straight forward.

Synchronization implies some amount of overhead. When a Regex is used from a single thread, this overhead is negligible. When a Regex is used from multiple threads simultaneously, it is possible for the overhead of synchronization from contention to impact performance. The specific cases where contention may happen is if you are calling any of these methods repeatedly from multiple threads simultaneously:

• shortest_match
• is_match
• find
• captures

In particular, every invocation of one of these methods must synchronize with other threads to retrieve its mutable scratch space before searching can start. If, however, you are using one of these methods:

• find_iter
• captures_iter

Then you may not suffer from contention since the cost of synchronization is amortized on construction of the iterator. That is, the mutable scratch space is obtained when the iterator is created and retained throughout its lifetime.

Only ask for what you need

Advice: Prefer in this order: is_match, find, captures.

There are three primary search methods on a Regex:

• is_match
• find
• captures

In general, these are ordered from fastest to slowest.

is_match is fastest because it doesn't actually need to find the start or the end of the leftmost-first match. It can quit immediately after it knows there is a match. For example, given the regex a+ and the haystack, aaaaa, the search will quit after examing the first byte.

In constrast, find must return both the start and end location of the leftmost-first match. It can use the DFA matcher for this, but must run it forwards once to find the end of the match and then run it backwards to find the start of the match. The two scans and the cost of finding the real end of the leftmost-first match make this more expensive than is_match.

captures is the most expensive of them all because it must do what find does, and then run either the bounded backtracker or the Pike VM to fill in the capture group locations. Both of these are simulations of an NFA, which must spend a lot of time shuffling states around. The DFA limits the performance hit somewhat by restricting the amount of text that must be searched via an NFA simulation.

One other method not mentioned is shortest_match. This method has precisely the same performance characteristics as is_match, except it will return the end location of when it discovered a match. For example, given the regex a+ and the haystack aaaaa, shortest_match may return 1 as opposed to 5, the latter of which being the correct end location of the leftmost-first match.

Literals in your regex may make it faster

Advice: Literals can reduce the work that the regex engine needs to do. Use them if you can, especially as prefixes.

In particular, if your regex starts with a prefix literal, the prefix is quickly searched before entering the (much slower) regex engine. For example, given the regex foo\w+, the literal foo will be searched for using Boyer-Moore. If there's no match, then no regex engine is ever used. Only when there's a match is the regex engine invoked at the location of the match, which effectively permits the regex engine to skip large portions of a haystack. If a regex is comprised entirely of literals (possibly more than one), then it's possible that the regex engine can be avoided entirely even when there's a match.

When one literal is found, Boyer-Moore is used. When multiple literals are found, then an optimized version of Aho-Corasick is used.

This optimization is in particular extended quite a bit in this crate. Here are a few examples of regexes that get literal prefixes detected:

• (foo|bar) detects foo and bar
• (a|b)c detects ac and bc
• [ab]foo[yz] detects afooy, afooz, bfooy and bfooz
• a?b detects a and b
• a*b detects a and b
• (ab){3,6} detects ababab

Literals in anchored regexes can also be used for detecting non-matches very quickly. For example, ^foo\w+ and \w+foo$ may be able to detect a non-match just by examing the first (or last) three bytes of the haystack.

Unicode word boundaries may prevent the DFA from being used

Advice: In most cases, \b should work well. If not, use (?-u:\b) instead of \b if you care about consistent performance more than correctness.

It's a sad state of the current implementation. At the moment, the DFA will try to interpret Unicode word boundaries as if they were ASCII word boundaries. If the DFA comes across any non-ASCII byte, it will quit and fall back to an alternative matching engine that can handle Unicode word boundaries correctly. The alternate matching engine is generally quite a bit slower (perhaps by an order of magnitude). If necessary, this can be ameliorated in two ways.

The first way is to add some number of literal prefixes to your regular expression. Even though the DFA may not be used, specialized routines will still kick in to find prefix literals quickly, which limits how much work the NFA simulation will need to do.

The second way is to give up on Unicode and use an ASCII word boundary instead. One can use an ASCII word boundary by disabling Unicode support. That is, instead of using \b, use (?-u:\b). Namely, given the regex \b.+\b, it can be transformed into a regex that uses the DFA with (?-u:\b).+(?-u:\b). It is important to limit the scope of disabling the u flag, since it might lead to a syntax error if the regex could match arbitrary bytes. For example, if one wrote (?-u)\b.+\b, then a syntax error would be returned because . matches any byte when the Unicode flag is disabled.

The second way isn't appreciably different than just using a Unicode word boundary in the first place, since the DFA will speculatively interpret it as an ASCII word boundary anyway. The key difference is that if an ASCII word boundary is used explicitly, then the DFA won't quit in the presence of non-ASCII UTF-8 bytes. This results in giving up correctness in exchange for more consistent performance.

N.B. When using bytes::Regex, Unicode support is disabled by default, so one can simply write \b to get an ASCII word boundary.

Excessive counting can lead to exponential state blow up in the DFA

Advice: Don't write regexes that cause DFA state blow up if you care about match performance.

Wait, didn't I say that this crate guards against exponential worst cases? Well, it turns out that the process of converting an NFA to a DFA can lead to an exponential blow up in the number of states. This crate specifically guards against exponential blow up by doing two things:

1. The DFA is computed lazily. That is, a state in the DFA only exists in memory if it is visited. In particular, the lazy DFA guarantees that at most one state is created for every byte of input. This, on its own, guarantees linear time complexity.
2. Of course, creating a new state for every byte of input means that search will go incredibly slow because of very large constant factors. On top of that, creating a state for every byte in a large haystack could result in exorbitant memory usage. To ameliorate this, the DFA bounds the number of states it can store. Once it reaches its limit, it flushes its cache. This prevents reuse of states that it already computed. If the cache is flushed too frequently, then the DFA will give up and execution will fall back to one of the NFA simulations.

In effect, this crate will detect exponential state blow up and fall back to a search routine with fixed memory requirements. This does, however, mean that searching will be much slower than one might expect. Regexes that rely on counting in particular are strong aggravators of this behavior. For example, matching [01]*1[01]{20}$ against a random sequence of 0s and 1s.

In the future, it may be possible to increase the bound that the DFA uses, which would allow the caller to choose how much memory they're willing to spend.

Resist the temptation to "optimize" regexes

Advice: This ain't a backtracking engine.

An entire book was written on how to optimize Perl-style regular expressions. Most of those techniques are not applicable for this library. For example, there is no problem with using non-greedy matching or having lots of alternations in your regex.