How to capitalize first letter of first word in a sentence?

Question

I am trying to write a function to clean up user input.

I am not trying to make it perfect. I would rather have a few names and acronyms in lowercase than a full paragraph in uppercase.

I think the function should use regular expressions but I'm pretty bad with those and I need some help.

If the following expressions are followed by a letter, I want to make that letter uppercase.

 "."
 ". " (followed by a space)
 "!"
 "! " (followed by a space)
 "?"
 "? " (followed by a space)

Even better, the function could add a space after ".", "!" and "?" if those are followed by a letter.

How this can be achieved?

Answer 1

How about this? Without Regex.

$letters = array(
'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'
);
foreach ($letters as $letter) {
    $string = str_replace('. ' . $letter, '. ' . ucwords($letter), $string);
    $string = str_replace('? ' . $letter, '? ' . ucwords($letter), $string);
    $string = str_replace('! ' . $letter, '! ' . ucwords($letter), $string);
}

Worked fine for me.

Answer 2

$output = preg_replace('/([.!?])\s*(\w)/e', "strtoupper('\\1 \\2')", ucfirst(strtolower($input)));

Since the modifier e is deprecated in PHP 5.5.0:

$output = preg_replace_callback('/([.!?])\s*(\w)/', function ($matches) {
    return strtoupper($matches[1] . ' ' . $matches[2]);
}, ucfirst(strtolower($input)));