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Creating Regex Patterns

Regular expressions (regex) are patterns used to match character combinations in strings. In JavaScript, regex patterns are created either with /pattern/flags syntax or the RegExp constructor.

Illustration of Creating Regex Patterns
Illustration of Creating Regex Patterns

Basic Syntax for Regex Patterns

  • /pattern/ — A regex literal enclosed between slashes.
  • new RegExp('pattern', 'flags') — A constructor that creates a regex object from a string pattern.
Creating Regex Patterns
MethodExampleNotes
Regex Literal/abc/Simple and concise, supports flags like i, g.
RegExp Constructornew RegExp('abc', 'i')Allows dynamic patterns but requires escaping backslashes.

Common Regex Components

  • abc — Matches the exact characters "abc".
  • . — Matches any single character except newline.
  • \d — Matches any digit (0–9).
  • \w — Matches any word character (letters, digits, underscore).
  • \s — Matches any whitespace (spaces, tabs).
  • [abc] — Matches any one character inside the brackets.
  • [^abc] — Matches any character not inside the brackets.
  • a* — Matches zero or more of the preceding element.
  • a+ — Matches one or more of the preceding element.
  • a? — Matches zero or one of the preceding element (optional).
  • {n} — Matches exactly n occurrences.
  • {n,m} — Matches between n and m occurrences.

💡 Regex Flags

Flags modify how the pattern behaves:
g — global search (find all matches),
i — case-insensitive,
m — multiline mode.

Escaping Special Characters

Characters like ., ?, *, +, ^, $, {}, [], (), and | have special meanings. To match them literally, prefix with a backslash \.

📌 Deep Dive: Creating a Simple Regex Pattern

JAVASCRIPT
// Literal regex to match 'cat' case-insensitively
const regex = /cat/i;

// Using RegExp constructor for dynamic pattern
const word = "dog";
const dynamicRegex = new RegExp(word, 'i');

// Test strings
console.log(regex.test('Cat'));       // true
console.log(dynamicRegex.test('DOG')); // true
Output
true
true

⚠️ Important

When using the RegExp constructor, backslashes must be escaped twice. For example, to match a digit: new RegExp('\\d').

Summary: Regex Creation

  • Use /pattern/flags for static regex patterns.
  • Use new RegExp('pattern', 'flags') for dynamic patterns.
  • Escape special characters to match them literally.
  • Apply flags (g, i, m) to modify matching behavior.
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