How to Convert camelCase to snake_case (Python, JavaScript & Online)

Converting userName to user_name is easy. Converting parseHTTPResponse correctly — and renaming variables inside real code without breaking it — takes a little more care. Here is how to do both.

The one-liner everyone uses (and where it fails)

The most common Python answer inserts an underscore before every capital letter:

import re

def camel_to_snake(name):
    return re.sub(r'(?<!^)(?=[A-Z])', '_', name).lower()

That works for userName → user_name, but splits acronyms letter by letter:

Input	One-liner	Expected
userName	user_name	user_name
parseHTTPResponse	parse_h_t_t_p_response	parse_http_response
getUserID	get_user_i_d	get_user_id

Python: handle acronyms with two passes

Split before a capitalised word first, then between a lowercase letter or digit and a capital:

import re

def camel_to_snake(name):
    name = re.sub(r'(.)([A-Z][a-z]+)', r'\1_\2', name)
    return re.sub(r'([a-z0-9])([A-Z])', r'\1_\2', name).lower()

camel_to_snake("parseHTTPResponse")  # 'parse_http_response'
camel_to_snake("getUserID")          # 'get_user_id'
camel_to_snake("base64Encode")       # 'base64_encode'

And back again:

def snake_to_camel(name):
    first, *rest = name.split('_')
    return first + ''.join(word.title() for word in rest)

snake_to_camel("max_retry_count")  # 'maxRetryCount'

If you'd rather use a library, inflection.underscore() and inflection.camelize(name, False) from the inflection package do the same.

JavaScript

const camelToSnake = (name) =>
  name
    .replace(/([A-Z]+)([A-Z][a-z])/g, "$1_$2")
    .replace(/([a-z0-9])([A-Z])/g, "$1_$2")
    .toLowerCase();

const snakeToCamel = (name) =>
  name.replace(/_([a-z0-9])/g, (_, c) => c.toUpperCase());

camelToSnake("parseHTTPResponse"); // "parse_http_response"
snakeToCamel("max_retry_count");   // "maxRetryCount"

Lodash's _.snakeCase() and _.camelCase() also work. Note that Lodash treats numbers as separate words, so check names like base64Encode.

Online: convert a list of names

The Case Converter converts names one per line to snake_case, camelCase, PascalCase, kebab-case or CONSTANT_CASE, keeping acronyms and digits together.

Renaming variables inside real code

Converting a name is easy. Renaming it everywhere in a file is where things break. A find-and-replace will also change:

• Strings — "userName" in an error message or a JSON key
• Attributes — response.userName, which belongs to another object
• Keyword arguments — fetch(pageSize=pageSize); the left side is the callee's parameter name
• Object keys — { totalCount } in JavaScript is shorthand for { totalCount: totalCount }, so renaming it changes your API

In an IDE, use rename refactoring (F2 in VS Code, Shift+F6 in JetBrains IDEs), which understands scope. For a snippet, the Case Converter's Code mode renames variables while skipping strings, attributes, keyword arguments and object keys, and lists anything it skipped so you can decide.

Which convention should you use?

Language	Variables & functions	Classes	Constants
Python	snake_case	PascalCase	CONSTANT_CASE
JavaScript / TypeScript	camelCase	PascalCase	CONSTANT_CASE or camelCase
Java	camelCase	PascalCase	CONSTANT_CASE
C / C++	project-dependent	project-dependent	CONSTANT_CASE

Whatever the language, consistency inside a project matters more than the convention itself — which is also why AI-generated code that mixes styles stands out in review. The Code Humanizer can bring it in line.