RegExp.prototype.test()

str

The string against which to match the regular expression. All values are coerced to strings, so omitting it or passing undefined causes test() to search for the string "undefined", which is rarely what you want.

true if there is a match between the regular expression and the string str. Otherwise, false.

Use test() whenever you want to know whether a pattern is found in a string. test() returns a boolean, unlike the String.prototype.search() method (which returns the index of a match, or -1 if not found).

To get more information (but with slower execution), use the exec() method. (This is similar to the String.prototype.match() method.)

As with exec() (or in combination with it), test() called multiple times on the same global regular expression instance will advance past the previous match.

Simple example that tests if "hello" is contained at the very beginning of a string, returning a boolean result.

const str = "hello world!";
const result = /^hello/.test(str);

console.log(result); // true

The following example logs a message which depends on the success of the test:

function testInput(re, str) {
  const midString = re.test(str) ? "contains" : "does not contain";
  console.log(`${str} ${midString} ${re.source}`);
}

When a regex has the global flag set, test() will advance the lastIndex of the regex. (RegExp.prototype.exec() also advances the lastIndex property.)

Further calls to test(str) will resume searching str starting from lastIndex. The lastIndex property will continue to increase each time test() returns true.

When test() returns false, the calling regex's lastIndex property will reset to 0.

The following example demonstrates this behavior:

const regex = /foo/g; // the "global" flag is set

// regex.lastIndex is at 0
regex.test("foo"); // true

// regex.lastIndex is now at 3
regex.test("foo"); // false

// regex.lastIndex is at 0
regex.test("barfoo"); // true

// regex.lastIndex is at 6
regex.test("foobar"); // false

// regex.lastIndex is at 0
regex.test("foobarfoo"); // true

// regex.lastIndex is at 3
regex.test("foobarfoo"); // true

// regex.lastIndex is at 9
regex.test("foobarfoo"); // false

// regex.lastIndex is at 0
// (...and so on)

• Regular expressions guide
• RegExp