How to check whether a string matches a regex in JS ?

To check if a string matches a regular expression (regex) in JavaScript, you can use the RegExp.test() method or the String.match() method. Below is a detailed guide with examples and best practices:

1. Using RegExp.test()

The test() method returns true if the regex matches the string, and false otherwise.

Syntax:

const regex = /pattern/;
const result = regex.test(string);

Examples:

1. Check if a string contains only letters:

   const regex = /^[A-Za-z]+$/;
   console.log(regex.test("Hello"));   // true
   console.log(regex.test("Hello123")); // false

2. Case-insensitive match (using the i flag):

   const regex = /hello/i; // 'i' flag ignores case
   console.log(regex.test("HELLO")); // true

3. Check for a date format (YYYY-MM-DD):

   const dateRegex = /^\d{4}-\d{2}-\d{2}$/;
   console.log(dateRegex.test("2023-10-05")); // true
   console.log(dateRegex.test("05-10-2023")); // false

2. Using String.match()

The match() method returns an array of matches or null if no matches are found.

Syntax:

const matches = string.match(/pattern/);
const isMatch = matches !== null; // Convert to boolean

Examples:

1. Check if a string contains a number:

   const str = "Price: $200";
   const hasNumber = str.match(/\d+/) !== null;
   console.log(hasNumber); // true

2. Extract all digits from a string:

   const str = "Room 101, Floor 5";
   const digits = str.match(/\d+/g); // 'g' flag for global search
   console.log(digits); // ["101", "5"]

3. Case-sensitive email validation:

   const email = "user@example.com";
   const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
   console.log(emailRegex.test(email)); // true

3. Key Differences: test() vs. match()

Aspect	RegExp.test()	String.match()
Return Value	true/false	Array of matches or null
Use Case	Check if a pattern exists	Extract matched values
Global Flag (g)	Returns true once per match (stateful)	Returns all matches in an array
Performance	Faster for simple checks	Slower (returns full match data)

4. Common Regex Patterns

Validate a Phone Number:

const phoneRegex = /^(\+\d{1,3})?(\d{3}[-\s]?){2}\d{4}$/;
console.log(phoneRegex.test("123-456-7890")); // true

Check for a URL:

const urlRegex = /^(https?:\/\/)?([\w-]+\.)+[\w-]+(\/[\w-./?%&=]*)?$/;
console.log(urlRegex.test("https://www.example.com")); // true

Password Strength Check:

// At least 8 chars, 1 uppercase, 1 number, 1 special character
const passwordRegex = /^(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*]).{8,}$/;
console.log(passwordRegex.test("Passw0rd!")); // true

5. Edge Cases & Best Practices

1. Global Flag (g) Caveat:

• test() with the g flag updates the regex object’s lastIndex property, leading to unexpected results in loops.
• Example:
  

const regex = /a/g; 
console.log(regex.test("abc")); // true 
console.log(regex.test("abc")); // false (starts searching from index 1)

1. Case Insensitivity:
   Use the i flag to ignore case:

   const regex = /hello/i;
   console.log(regex.test("HELLO")); // true

3. Dynamic Regex Patterns:
   Use the RegExp constructor for dynamic patterns:

   const pattern = "example";
   const regex = new RegExp(pattern, "i"); // Case-insensitive
   console.log(regex.test("EXAMPLE")); // true

4. Special Characters:
   Escape regex-specific characters like ., *, and + with a backslash (\):

   const regex = /\.jpg$/; // Matches filenames ending with ".jpg"
   console.log(regex.test("image.jpg")); // true

Summary

• Use RegExp.test() for simple existence checks (returns true/false).
• Use String.match() to extract matched substrings.
• Always test your regex patterns thoroughly (e.g., RegExr).