Matching strings against regular expressions

String match: String.match()

The match() method in JavaScript retrieves the result of matching a string against a regular expression. It returns an array of matches or null if no matches are found.

Syntax:

string.match(regexp)

Parameters:

1. regexp: A regular expression object. If not a regular expression, it is implicitly converted to one using new RegExp(regexp).

Return Value:

The match() method returns an array with the matches, or null if no matches are found. The returned array has the following properties:

• If the regular expression doesn’t have the global flag (g), the array contains the first match found and any capturing groups.
• If the regular expression has the global flag (g), the array contains all matches but no capturing groups.

Examples:

Basic Usage:

const text = 'The rain in Spain stays mainly in the plain';
const matches = text.match(/ain/g);
console.log(matches); // Output: ["ain", "ain", "ain"]

In this example, the match() method finds all occurrences of ‘ain’ in the string and returns them as an array.

Without Global Flag:

const text = 'The rain in Spain stays mainly in the plain';
const match = text.match(/ain/);
console.log(match);
// Output: ["ain", index: 5, input: "The rain in Spain stays mainly in the plain", groups: undefined]

Without the global flag, match() returns an array with the first match and additional properties like the index where the match was found.

Using Capturing Groups:

const text = 'JavaScript was created in 1995';
const match = text.match(/(\w+) was created in (\d+)/);
console.log(match);
// Output: ["JavaScript was created in 1995", "JavaScript", "1995", index: 0, input: "JavaScript was created in 1995", groups: undefined]

The first element is the entire match, followed by each captured group.

No Match Case:

const text = 'Hello, world!';
const matches = text.match(/xyz/);
console.log(matches); // Output: null

When no matches are found, match() returns null.

Common Use Cases:

1. Extracting Information from Text:

function extractEmails(text) {
  const emailRegex = /[\w.-]+@[\w.-]+\.\w+/g;
  return text.match(emailRegex) || [];
}

const content = 'Contact us at support@example.com or sales@example.com for assistance.';
console.log(extractEmails(content));
// Output: ["support@example.com", "sales@example.com"]

2. Parsing Data from Formatted Text:

function parseCSVLine(line) {
  return line.match(/(".*?"|[^,]+)(?=,|$)/g)
    .map(item => item.replace(/^"|"$/g, ''));
}

const csvLine = 'John,"Doe,Jr",42,"New York, NY"';
console.log(parseCSVLine(csvLine));
// Output: ["John", "Doe,Jr", "42", "New York, NY"]

3. Validating Input Format:

function isValidUsername(username) {
  // Alphanumeric, 3-16 characters
  const match = username.match(/^[a-z0-9]{3,16}$/i);
  return match !== null;
}

console.log(isValidUsername('user123')); // Output: true
console.log(isValidUsername('us')); // Output: false (too short)
console.log(isValidUsername('user@123')); // Output: false (invalid character)

Using match() with String Methods:

The match() method is often combined with other string methods for more complex text processing:

function countWords(text) {
  const words = text.toLowerCase().match(/\b\w+\b/g) || [];
  return words.length;
}

const paragraph = 'The quick brown fox jumps over the lazy dog.';
console.log(countWords(paragraph)); // Output: 9

By using the String.match() method, you can perform powerful pattern matching and text extraction operations using regular expressions, which is essential for data validation, parsing, and text analysis in JavaScript applications.