String.prototype.match()

The match() method retrieves the result of matching a string against a regular expression.

regexp

A regular expression object.

If regexp is a non-RegExp object, it is implicitly converted to a RegExp by using new RegExp(regexp).

If you don't give any parameter and use the match() method directly, you will get an Array with an empty string: [""].

An Array whose contents depend on the presence or absence of the global (g) flag, or null if no matches are found.

• If the g flag is used, all results matching the complete regular expression will be returned, but capturing groups will not.
• if the g flag is not used, only the first complete match and its related capturing groups are returned. In this case, the returned item will have additional properties as described below.

Additional properties

As explained above, some results contain additional properties as described below.

groups

An object of named capturing groups whose keys are the names and values are the capturing groups or undefined if no named capturing groups were defined. See Groups and Ranges for more information.

index

The index of the search at which the result was found.

input

A copy of the search string.

If the regular expression does not include the g flag, str.match() will return the same result as RegExp.exec().

• If you need to know if a string matches a regular expression RegExp, use RegExp.test().
• If you only want the first match found, you might want to use RegExp.exec() instead.
• If you want to obtain capture groups and the global flag is set, you need to use RegExp.exec() or String.prototype.matchAll() instead.

In the following example, match() is used to find 'Chapter' followed by 1 or more numeric characters followed by a decimal point and numeric character 0 or more times.

The regular expression includes the i flag so that upper/lower case differences will be ignored.

const str = 'For more information, see Chapter 3.4.5.1';
const re = /see (chapter \d+(\.\d)*)/i;
const found = str.match(re);

console.log(found);

// logs [ 'see Chapter 3.4.5.1',
//        'Chapter 3.4.5.1',
//        '.1',
//        index: 22,
//        input: 'For more information, see Chapter 3.4.5.1' ]

// 'see Chapter 3.4.5.1' is the whole match.
// 'Chapter 3.4.5.1' was captured by '(chapter \d+(\.\d)*)'.
// '.1' was the last value captured by '(\.\d)'.
// The 'index' property (22) is the zero-based index of the whole match.
// The 'input' property is the original string that was parsed.

The following example demonstrates the use of the global and ignore case flags with match(). All letters A through E and a through e are returned, each its own element in the array.

const str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
const regexp = /[A-E]/gi;
const matches_array = str.match(regexp);

console.log(matches_array);
// ['A', 'B', 'C', 'D', 'E', 'a', 'b', 'c', 'd', 'e']

In browsers which support named capturing groups, the following code captures "fox" or "cat" into a group named "animal":

const paragraph = 'The quick brown fox jumps over the lazy dog. It barked.';

const capturingRegex = /(?<animal>fox|cat) jumps over/;
const found = paragraph.match(capturingRegex);
console.log(found.groups); // {animal: "fox"}

When the regexp parameter is a string or a number, it is implicitly converted to a RegExp by using new RegExp(regexp).

If it is a positive number with a positive sign, RegExp() will ignore the positive sign.

const str1 = "NaN means not a number. Infinity contains -Infinity and +Infinity in JavaScript.",
    str2 = "My grandfather is 65 years old and My grandmother is 63 years old.",
    str3 = "The contract was declared null and void.";
str1.match("number");   // "number" is a string. returns ["number"]
str1.match(NaN);        // the type of NaN is the number. returns ["NaN"]
str1.match(Infinity);   // the type of Infinity is the number. returns ["Infinity"]
str1.match(+Infinity);  // returns ["Infinity"]
str1.match(-Infinity);  // returns ["-Infinity"]
str2.match(65);         // returns ["65"]
str2.match(+65);        // A number with a positive sign. returns ["65"]
str3.match(null);       // returns ["null"]

• String.prototype.matchAll()
• RegExp
• RegExp.prototype.exec()
• RegExp.prototype.test()