Matt Matt - 1 year ago 133
PHP Question

Using preg_replace() to convert CamelCase to snake_case

I have a method now that will convert my camel case strings to snake case, but it's broken into three calls of


public function camelToUnderscore($string, $us = "-")
// insert hyphen between any letter and the beginning of a numeric chain
$string = preg_replace('/([a-z]+)([0-9]+)/i', '$1'.$us.'$2', $string);
// insert hyphen between any lower-to-upper-case letter chain
$string = preg_replace('/([a-z]+)([A-Z]+)/', '$1'.$us.'$2', $string);
// insert hyphen between the end of a numeric chain and the beginning of an alpha chain
$string = preg_replace('/([0-9]+)([a-z]+)/i', '$1'.$us.'$2', $string);

// Lowercase
$string = strtolower($string);

return $string;

I wrote tests to verify its accuracy, and it works properly with the following array of inputs (
array('input' => 'output')

$test_values = [
'foo' => 'foo',
'fooBar' => 'foo-bar',
'foo123' => 'foo-123',
'123Foo' => '123-foo',
'fooBar123' => 'foo-bar-123',
'foo123Bar' => 'foo-123-bar',
'123FooBar' => '123-foo-bar',

I'm wondering if there's a way to reduce my
calls to a single line which will give me the same result. Any ideas?

NOTE: Referring to this post, my research has shown me a
regex that gets me almost the result I want, except it doesn't work on the example of
to convert it to

Answer Source

You can use lookarounds to do all this in a single regex:

function camelToUnderscore($string, $us = "-") {
    return strtolower(preg_replace(
        '/(?<=\d)(?=[A-Za-z])|(?<=[A-Za-z])(?=\d)|(?<=[a-z])(?=[A-Z])/', $us, $string));

RegEx Demo

Code Demo

RegEx Description:

(?<=\d)(?=[A-Za-z])  # if previous position has a digit and next has a letter
|                    # OR
(?<=[A-Za-z])(?=\d)  # if previous position has a letter and next has a digit
|                    # OR
(?<=[a-z])(?=[A-Z])  # if previous position has a lowercase and next has a uppercase letter
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