Using preg_replace() to convert CamelCase to snake_case

Question

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

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 preg_replace() 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 preg_replace() regex that gets me almost the result I want, except it doesn't work on the example of foo123 to convert it to foo-123.

Answer 1

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

Answer 2

Here is my two cents based on the duplicated post I flagged earlier. The accepted solution here is awesome. I just wanted to try to solve it with what was shared :

function camelToUnderscore($string, $us = "-") {
    return strtolower(preg_replace('/(?<!^)[A-Z]+|(?<!^|\d)[\d]+/', $us.'$0', $string));
}

Example :

Array
(
    [0] => foo
    [1] => fooBar
    [2] => foo123
    [3] => 123Foo
    [4] => fooBar123
    [5] => foo123Bar
    [6] => 123FooBar
)

foreach ($arr as $item) {
    echo camelToUnderscore($item);
    echo "\r\n";
}

Output :

foo
foo-bar
foo-123
123-foo
foo-bar-123
foo-123-bar
123-foo-bar

Explanation :

(?<!^)[A-Z]+      // Match one or more Capital letter not at start of the string
|                 // OR
(?<!^|\d)[\d]+    // Match one or more digit not at start of the string

$us.'$0'          // Substitute the matching pattern(s)

online regex

The question is already solved so I won't say that I hope it helps but maybe someone will find this useful.

---

EDIT

There are limits with this regex :

foo123bar => foo-123bar
fooBARFoo => foo-barfoo

Thanks to @urban for pointed it out. Here is his link with tests with the three solutions posted on this question :

three solutions demo

Answer 3

From a colleague:

$string = preg_replace(array($pattern1, $pattern2), $us.'$1', $string); might work

My solution:

public function camelToUnderscore($string, $us = "-")
{
    $patterns = [
        '/([a-z]+)([0-9]+)/i',
        '/([a-z]+)([A-Z]+)/',
        '/([0-9]+)([a-z]+)/i'
    ];
    $string = preg_replace($patterns, '$1'.$us.'$2', $string);

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

    return $string;
}