How to swap uppercase and lowercase letters in PHP using regular expressions

In PHP, regular expressions are a powerful tool that can help us process text quickly. One common task is to swap uppercase and lowercase letters. This can be useful in certain situations, such as when we need to convert a section of text to first letter upper case or all upper case or all lower case. In this article, we'll show you how to do this conversion in PHP using regular expressions.

1. The principle of swapping uppercase and lowercase letters

In PHP, to swap the uppercase and lowercase letters of a character, you can use the strtoupper() function to convert lowercase letters to uppercase letters, or Use the strtolower() function to convert uppercase letters to lowercase letters. But if you want to convert the entire string to the opposite case, you need to use regular expressions.

To realize the function of exchanging upper and lower case in a string, you need to use the capture and replacement functions of regular expressions. Specifically, we need to write a regular expression that will match every lowercase and uppercase letter in the string. We can then use the preg_replace_callback() function to capture these letters into a callback function, swap their cases and replace them back into the string.

2. Implementation code

The following is a simple PHP function for exchanging uppercase and lowercase letters in a string:

function swap_case($string) {
    return preg_replace_callback('/[a-zA-Z]/', function($matches) {
        $char = $matches[0];
        if (ctype_upper($char)) {
            return strtolower($char);
        } else {
            return strtoupper($char);
        }
    }, $string);
}

This function defines a name Is the function of swap_case(), which receives a string as a parameter. In the function, we use the preg_replace_callback() function to match every lowercase and uppercase letter in the string. Specifically, we use the regular expression /[a-zA-Z]/ to match all letters. Then, capture each letter matched into a callback function, use the ctype_upper() function to check its case, convert it to uppercase in the case of a lowercase letter, and convert it to lowercase letters and returns the converted character to the replaced position.

Here are some examples showing how to use this function:

echo swap_case("Hello world!");  // 输出 "hELLO WORLD!"
echo swap_case("The quick brown fox jumps over the lazy dog."); // 输出 "tHE QUICK BROWN FOX JUMPS OVER THE LAZY DOG."

As shown in the above example, the uppercase and lowercase letters in the input string have been swapped and output to the screen.

3. Summary

In this article, we introduced the method of using regular expressions to interchange uppercase and lowercase letters in PHP. We learned about the capture and replacement capabilities of regular expressions and combined them using the preg_replace_callback() function. Finally, we wrote a simple function to swap string case and showed some examples. Hope this article helps you!

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