Replace a String case-insensitive in JavaScript

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Borislav Hadzhiev

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Replace a String case-insensitive in JavaScript - String.replace() #

To do a case-insensitive string replacement in JavaScript:

  1. Call the String.replace() method method on the string.
  2. Set the ignore flag on the first argument.
  3. The ignore flag does a case-insensitive search in the string.
index.js
const str = 'HELLO HELLO world'; const replaced = str.replace(/hello/gi, 'bye'); console.log(replaced); // ๐Ÿ‘‰๏ธ bye bye world

We used the String.replace method to replace all occurrences of the string hello with the string bye.

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:

NameDescription
patternThe pattern to look for in the string. Can be a string or a regular expression.
replacementA string used to replace the substring match by the supplied pattern.

The first argument we passed to the replace() method is a regular expression.

index.js
const str = 'HELLO HELLO world'; const replaced = str.replace(/hello/gi, 'bye'); console.log(replaced); // ๐Ÿ‘‰๏ธ bye bye world

Note that we set the i and g flags on the regular expression.

The i flag stands for ignore and does a case-insensitive search in string.

The g flag stands for global and replaces all occurrences of the matched string.

If you only want to do a case-insensitive replacement the first time the regular expression matches the string, remove the g flag:

index.js
const str = 'HELLO HELLO world'; const replacedOnce = str.replace(/hello/i, 'bye'); console.log(replacedOnce); // ๐Ÿ‘‰๏ธ bye HELLO world

The regular expression is matched in the string two times, however, we haven't set the g flag so only the first match is replaced.

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.

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