RegExp.prototype.test()

The test() method of RegExp instances executes a search with this regular expression for a match between a regular expression and a specified string. Returns true if there is a match; false otherwise.

JavaScript RegExp objects are stateful when they have the global or sticky flags set (e.g., /foo/g or /foo/y). They store a lastIndex from the previous match. Using this internally, test() can be used to iterate over multiple matches in a string of text (with capture groups).

Syntax

test(str)

Parameters

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.

Return value

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

Description

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.

Examples

Using test()

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}`);
}

Using test() on a regex with the "global" flag

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.

Note: As long as test() returns true, lastIndex will not reset—even when testing a different string!

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)