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JavaScript RegExp Reference

Syntax

A regular expression is created with the pattern and optional modifiers:

// Syntax
let pattern = /Technology/i; // 'i' modifier: case-insensitive match
Example: let pattern = /Technology/i;

Creating a RegExp

You can create regular expressions using literals or the RegExp constructor.

// Using literal syntax
const regexLiteral = /abc/;

// Using the RegExp constructor
const regexConstructor = new RegExp('abc');
Output: RegExp created using literal and constructor

Modifiers

Modifiers adjust the behavior of a RegExp pattern:

  • g: Global match
  • i: Case-insensitive
  • m: Multiline match

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <Script>
        const regex = new RegExp(/^a...s$/i);
        console.log(regex.test('Alias')); // true
    </Script>
</body>
</html>
Output: true

Brackets

Brackets define character classes:

  • [abc]: Matches any character in abc
  • [^abc]: Matches any character not in abc
  • [0-9]: Matches any digit

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <Script>
        const digitRegex = /[0-9]/;
        console.log(digitRegex.test('abc123')); // true
    </Script>
</body>
</html>
Output: true

Quantifiers

Quantifiers specify the number of occurrences:

  • *: Matches zero or more times
  • +: Matches one or more times
  • {n,m}: Matches between n and m times

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <Script>
       const quantifierRegex = /\d{2,4}/;
       console.log(quantifierRegex.test('12345')); // true
    </Script>
</body>
</html>
Output: true

Anchors

Anchors specify position:

  • ^: Matches start of string
  • $: Matches end of string

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <Script>
            const anchorRegex = /^start/;
            console.log(anchorRegex.test('start with regex')); // true
    </Script>
</body>
</html>
Output: true

Escape Characters

Escape characters to match symbols:


<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <Script>
        const escapeRegex = /\d+\.\d+/;
        console.log(escapeRegex.test('3.14')); // true
    </Script>
</body>
</html>
Output: true

Assertions

Assertions are used to match specific conditions:

  • (?=...): Positive lookahead
  • (?!...): Negative lookahead

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <Script>
        const lookaheadRegex = /\w+(?=\s)/;
        console.log('Word followed by space'.match(lookaheadRegex)); // ["Word"]
    </Script>
</body>
</html>
Output: ["Word"]
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