re2-wasm

This is not an officially supported Google product.

This README is modified from the node-re2 README, licensed under The "New" BSD License

This project provides bindings for RE2: fast, safe alternative to backtracking regular expression engines written by Russ Cox. To learn more about RE2, start with an overview Regular Expression Matching in the Wild. More resources can be found at his Implementing Regular Expressions page.

RE2's regular expression language is almost a superset of what is provided by RegExp (see Syntax), but it lacks two features: backreferences and lookahead assertions. See below for more details.

RE2 object emulates standard RegExp making it a practical drop-in replacement in most cases. RE2 is extended to provide String-based regular expression methods as well. To help to convert RegExp objects to RE2 its constructor can take RegExp directly honoring all properties.

Why use re2-wasm?

The built-in Node.js regular expression engine can run in exponential time with a special combination:

• A vulnerable regular expression
• "Evil input"

This can lead to what is known as a Regular Expression Denial of Service (ReDoS). To tell if your regular expressions are vulnerable, you might try the one of these projects:

• rxxr2
• safe-regex

However, neither project is perfect.

re2-wasm can protect your Node.js application from ReDoS. re2-wasm makes vulnerable regular expression patterns safe by evaluating them in RE2 instead of the built-in Node.js regex engine.

Standard features

RE2 object can be created just like RegExp:

• new RE2(pattern[, flags])

Supported properties:

• re2.lastIndex
• re2.global
• re2.ignoreCase
• re2.multiline
• re2.unicode
  • RE2 engine always works in the Unicode mode. See details below.
• re2.sticky
• re2.source
• re2.flags

Supported methods:

• re2.exec(str)
• re2.test(str)
• re2.toString()

The following well-known symbol-based methods are supported (see Symbols):

• re2[Symbol.match](str)
• re2[Symbol.search](str)
• re2[Symbol.replace](str, newSubStr|function)
• re2[Symbol.split](str[, limit])

It allows to use RE2 instances on strings directly, just like RegExp instances:

var re = new RE2("1", 'u');
"213".match(re);        // [ '1', index: 1, input: '213' ]
"213".search(re);       // 1
"213".replace(re, "+"); // 2+3
"213".split(re);        // [ '2', '3' ]

Named groups are supported.

Extensions

Shortcut construction

RE2 object can be created from a regular expression:

var re1 = new RE2(/ab*/igu); // from a RegExp object
var re2 = new RE2(re1);     // from another RE2 object

String methods

Standard String defines four more methods that can use regular expressions. RE2 provides them as methods exchanging positions of a string, and a regular expression:

• re2.match(str)
  • See str.match(regexp)
• re2.replace(str, newSubStr|function)
  • See str.replace(regexp, newSubStr|function)
• re2.search(str)
  • See str.search(regexp)
• re2.split(str[, limit])
  • See str.split(regexp[, limit])

Property: internalSource

Starting 1.8.0 property source emulates the same property of RegExp, meaning that it can be used to create an identical RE2 or RegExp instance. Sometimes, for troubleshooting purposes, a user wants to inspect a RE2 translated source. It is available as a read-only property called internalSource.

Unicode Mode

The RE2 engine only works in Unicode mode, so the RE2 class must always be constructed with the u flag to enable unicode mode.

How to install

Installation:

npm install --save re2-wasm

How to use

It is used just like a RegExp object.

var { RE2 } = require("re2-wasm");

// with default flags
var re = new RE2("a(b*)", 'u');
var result = re.exec("abbc");
console.log(result[0]); // "abb"
console.log(result[1]); // "bb"

result = re.exec("aBbC");
console.log(result[0]); // "a"
console.log(result[1]); // ""

// with explicit flags
re = new RE2("a(b*)", "iu");
result = re.exec("aBbC");
console.log(result[0]); // "aBb"
console.log(result[1]); // "Bb"

// from regular expression object
var regexp = new RegExp("a(b*)", "iu");
re = new RE2(regexp);
result = re.exec("aBbC");
console.log(result[0]); // "aBb"
console.log(result[1]); // "Bb"

// from regular expression literal
re = new RE2(/a(b*)/iu);
result = re.exec("aBbC");
console.log(result[0]); // "aBb"
console.log(result[1]); // "Bb"

// from another RE2 object
var rex = new RE2(re);
result = rex.exec("aBbC");
console.log(result[0]); // "aBb"
console.log(result[1]); // "Bb"

// shortcut
result = new RE2("ab*", 'u').exec("abba");

Limitations (things RE2 does not support)

RE2 consciously avoids any regular expression features that require worst-case exponential time to evaluate. These features are essentially those that describe a Context-Free Language (CFL) rather than a Regular Expression, and are extensions to the traditional regular expression language because some people don't know when enough is enough.

The most noteworthy missing features are backreferences and lookahead assertions. If your application uses these features, you should continue to use RegExp. But since these features are fundamentally vulnerable to ReDoS, you should strongly consider replacing them.

RE2 will throw a SyntaxError if you try to declare a regular expression using these features. If you are evaluating an externally-provided regular expression, wrap your RE2 declarations in a try-catch block. It allows to use RegExp, when RE2 misses a feature:

var re = /(a)+(b)*/u;
try {
  re = new RE2(re);
  // use RE2 as a drop-in replacement
} catch (e) {
  // suppress an error, and use
  // the original RegExp
}
var result = re.exec(sample);

In addition to these missing features, RE2 also behaves somewhat differently from the built-in regular expression engine in corner cases.

Backreferences

RE2 doesn't support backreferences, which are numbered references to previously matched groups, like so: \1, \2, and so on. Example of backrefrences:

/(cat|dog)\1/.test("catcat"); // true
/(cat|dog)\1/.test("dogdog"); // true
/(cat|dog)\1/.test("catdog"); // false
/(cat|dog)\1/.test("dogcat"); // false

Lookahead assertions

RE2 doesn't support lookahead assertions, which are ways to allow a matching dependent on subsequent contents.

/abc(?=def)/; // match abc only if it is followed by def
/abc(?!def)/; // match abc only if it is not followed by def

Mismatched behavior

RE2 and the built-in regex engines disagree a bit. Before you switch to RE2, verify that your regular expressions continue to work as expected. They should do so in the vast majority of cases.

Here is an example of a case where they may not:

var { RE2 }  = require("re2-wasm");

var pattern = '(?:(a)|(b)|(c))+';

var built_in = new RegExp(pattern);
var re2 = new RE2(pattern, 'u');

var input = 'abc';

var bi_res = built_in.exec(input);
var re2_res = re2.exec(input);

console.log('bi_res: ' + bi_res);    // prints: bi_res: abc,,,c
console.log('re2_res : ' + re2_res); // prints: re2_res : abc,a,b,c

Unicode

RE2 only works in the Unicode mode. The u flag must be passed to the RE2 constructor.

License

Apache 2.0