The String `match()` Function in JavaScript
JavaScript Strings have a match() method that returns an array if the string matches a given regular expression, or null otherwise.
'abc'.match(/a/); // [ 'a', index: 0, input: 'abc', groups: undefined ]
'abc'.match(/z/); // null
// Use `match()` to check if a string matches a regexp
if (str.match(regexp) != null) {
// matches!
}match() is very similar to the RegExp test() method, which returns true if a regexp matches a given string.
/a/.test('abc'); // true
/z/.test('abc'); // falseIf all you want to do is test whether a string matches a regexp, you should use test() because test() is slightly faster.
However, match() has some helpful advanced features.
Capture Groups
The match() function's return value contains the regular expressions' capture groups.
A capture group is a subsection of the regular expression in parentheses ().
For example, \d is a capture group in the following.
const arr = 'There are 4 lights'.match(/(\d) lights/);
arr[0]; // "4 lights", the complete match
arr[1]; // "4", the first capture groupCapture groups are useful for pulling subsections of a given regular expression result.
For example, below is how you can use capture groups to convert a date in YYYY-MM-DD format into year, month, and day.
const str = '2022-06-01';
// 4 digits, dash, 2 digits, dash, 2 digits. The 4 digits, 2 digits, 2 digits
// are capture groups.
const [year, month, day] = str.match(/(\d{4})-(\d{2})-(\d{2})/).slice(1, 4);
year; // '2022'
month; // '06'
day; // '01'Note that capture groups do not work if you specify the g flag on your regular expression.
Practical Example: Standardizing US Telephone Numbers
There are many different ways to write a US telephone number. All the below examples are not uncommon.
2016930123
+12016930123
+1 201-693-0123
+1 (201)-693-0123
201 693 0123Suppose you want to standardize, and always store 2016930123, but allow all the other formats?
Use the following regular expression: optional leading +1 as a capture group, and then capture groups for the area code, telephone prefix, and line number.
/^(\+1\s?)?\(?([\d]{3})\)?[-. ]?([\d]{3})[-. ]?([\d]{4})$/Assuming there is a match, the following will convert a telephone number in any of the given formats to 2016930123
const processedTelephoneNumber = telephone.
match(/^(\+1\s?)?\(?([\d]{3})\)?[-. ]?([\d]{3})[-. ]?([\d]{4})$/).
slice(2, 5).
join('');