Replace/Remove characters that Don't match Regex in JS

# Replace/Remove characters that Don't match Regex

To replace or remove characters that don't match a regex, call the replace() method on the string passing it a regular expression that uses the caret ^ symbol, e.g. /[^a-z]+/.

The replace method will return a new string where the not matching characters are replaced or removed.

index.js

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const str = 'Hello123!@#';

// ✅ Remove not matching
const result1 = str.replace(/[^a-zA-Z]+/, '');
console.log(result1); // 👉️ Hello

// ✅ Replace not matching
const result2 = str.replace(/[^a-zA-Z]+/, '!');
console.log(result2); // 👉️ Hello!

// ✅ Replace each occurrence of not matching
const result3 = str.replace(/[^a-zA-Z]/g, '=');
console.log(result3); // 👉️ "Hello======"

replace remove characters that dont match regex

The code for this article is available on GitHub

The String.replace() method returns a new string with one, some, or all matches of a regular expression replaced with the provided replacement.

The method takes the following parameters:

Name	Description
pattern	The pattern to look for in the string. Can be a string or a regular expression.
replacement	A string used to replace the substring match by the supplied pattern.

The first example shows how to remove all characters that are not lowercase or uppercase Latin letters.

The forward slashes / / mark the beginning and end of the regular expression.

The square brackets [] are called a character class.

When used in a character class, the caret ^ means "not the following".

In our case, anything that is not a Latin letter in the range of a-z.

The + matches the preceding item (the character class) one or more times. In other words, we consider multiple non-lowercase or uppercase, Latin letters to be a single match.

For any character that is not a lowercase or uppercase Latin letter, we provided an empty string as a replacement to remove the character from the string.

The String.replace() method returns a new string with the matches of the pattern replaced. The method doesn't change the original string.

Strings are immutable in JavaScript.

# Replace the characters that don't match the regex

You can also use the String.replace() method to replace the characters that don't match the regular expression.

index.js

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const str = 'Hello123!@#';

const result = str.replace(/[^a-zA-Z]+/, '!');
console.log(result); // 👉️ Hello!

replace characters that dont match the regex

The code for this article is available on GitHub

The code sample replaces one or more occurrences of non-letters with an exclamation point.

In other words, we replace multiple non-matching characters next to one another with a single replacement.

# Replace each occurrence of a character that doesn't match the regex

If you need to replace each occurrence of a character that doesn't match the regex with a specific character, remove the plus + symbol from the regex.

index.js

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const str = 'abc123';

const result = str.replace(/[^a-z]/g, '!');
console.log(result); // 👉️ "abc!!!"

replace each occurrence of character that does not match regex

We used the g (global) flag because we want to match all occurrences of a non-lowercase, Latin letter, and not just the first occurrence.

If you only need to replace the first occurrence of a non-matching character, remove the g flag.

index.js

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const str = 'abc123';

const result3 = str.replace(/[^a-z]/, '!');
console.log(result3); // 👉️ "abc!23"

The code for this article is available on GitHub

In this example, we only replaced the first occurrence of a non-matching character. This is the default behavior of the replace method.

If you ever need help reading a regular expression, check out this regular expression cheat sheet by MDN.

It contains a table with the name and the meaning of each special character with examples.

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Replace/Remove characters that Don't match Regex in JS