[JavaScript] RegExp Object

The RegExp object is used for matching text with a pattern.

It introduced PCRE from ES3.

Create a Regular Expression

RegExp instance can be created by the constructor or literal notation.

Please see the 3 ways below.

const re = /ab+c/i;
const re = new RegExp('ab+c', 'i');
const re = new RegExp(/ab+c/, 'i'));

The pattern syntax is below.

/PATTERN/FLAG

/ symbol means to start and end.

In other words, a regular expression pattern should be between the //.

Flag

The flag is optional and functional.

Flag	Description	Corresponding Property
d	Generate indices for substring matches	hasIndices
g	Global search	global
i	Case-insensitive search	ignoreCase
m	Multi-line search	multiline
s	Allows . to match newline characters	dotAll
u	Treat a pattern as a sequence of Unicode code points	unicode
y	Perform a "sticky" search	sticky

Pattern

Quantifiers

Character	Meaning
*	Matches the preceding item 0 or more times
+	Matches the preceding item 1 or more times
?	Matches the preceding item 0 or 1 times
{n}	Matches exactly n occurrences of the preceding item Where n is a positive integer
{n,}	Matches at least n occurrences of the preceding item Where n is a positive integer
{n,m}	Matches at least n and at most m occurrences of the preceding item Where both n and m are a positive integer and m > n
*?, +?, ?? {n}?, {n,}?, {n,m}?	By default quantifiers are greedy The ? character after the quantifier makes the quantifier non-greedy

Assertions

Character	Meaning
^	Matches the beginning of input
$	Matches the end of input
\b	Matches a word boundary
\B	Matches a non-word boundary
x(?=y)	Matches x only if x is followed by y
x(?|y)	Matches x only if x is not followed by y
(?<=y)x	Matches x only if x is preceded by y
(?<!y)x	Matches x only if x is not preceded by y

Groups and Backreferences

Character	Meaning
(x)	Matches x and remembers the match
(?<Name>x)	Matches x and stores it on the groups property of the returned matches under the name specified by <Name>
(?:x)	Matches x but does not remember the match
\n	A back reference to the last substring matching the n parenthetical in the regular expression Where n is a positive integer
\k<Name>	A back reference to the last substring matching the named capture group specified by <Name>

Character Classes

Character	Meaning
[xyz] [a-z]	Matches any one of the closed characters We can specify a range of characters by using a hyphen
[^xyz] [^a-z]	Matches anything that is not enclosed in the brackets
.	Matches any single character except line terminators
\d	Matches any digit
\D	Matches any character that is not a digit
\w	Matches any alphanumeric character from the basic Latin alphabet, including the underscore.
\W	Matches any character that is not a word character from the basic Latin alphabet
\s	Matches a single white space character, including space, tab, form feed, line feed, and other Unicode spaces
\S	Matches a single character other than white space
\t	Matches a horizontal tab
\r	Matches a carriage return
\n	Matches a line feed
\v	Matches a vertical tab
\f	Matches a form feed
[\b]	Matches a backspace
\0	Matches a NUL character
\cX	Matches a control character using caret notation Where X is a letter from A-Z
\xhh	Matches the character with the code hh (2 hexadecimal digits)
\uhhhh	Matches a UTF-16 code-unit with the value hhhh (4 hexadecimal digits)
\u{hhhh} or \u{hhhhh}	Matches the character with the Unicode value U+hhhh or U+hhhhh (4 or 5 hexadecimal digits)
\	Indicates that the following character should be treated specially or escaped.
x|y	Matches either x or y

RegExp Method

exec()

This method executes a search for a match in a specified string.

It returns a result array or null.

const re = /pattern/;
re.exec('target string');

test()

This method executes a search for a match between a regular expression and a specified string.

It returns true or false.

const re = /pattern/;
re.test('target string');

toString()

This method returns a string representing the regular expression.

const re = /pattern/;
console.log(re.toString()); // /pattern/

match()

This method is the method of a String object.

It returns a result array or null.

'target string'.match(/pattern/);

Example

This example checks if the URL starts with HTTP or HTTPS.

const url = 'https://example.com';

/^https?:\/\//.test(url);

This example checks whether the cell phone number format fits.

const mobile = '010-1234-54678';

/^\d{3}-d{3,4}-\d{4}$/.test(mobile);

This example checks if the ID consists of 4 to 10 alphanumeric characters.

const id ='q1w2e3';

/^[A-Za-z0-9]{4,10}$/.test(id);