[php] [spin] Finding string and replacing with same case string

I'd suggest using preg_replace_callback with a callback function that examines the matched word to see if (a) the first letter is not capitalized, or (b) the first letter is the only capitalized letter, or (c) the first letter is not the only capitalized letter, and then replace with the properly modified replacement word as desired.

You can find your string and do two tests:

$outputString = 'hi';
if ( $foundString == ucfirst($foundString) ) {
   $outputString = ucfirst($outputString);
} else if ( $foundString == strtoupper($foundString) ) {
   $outputString = strtoupper($outputString);
} else {
   // do not modify string's case
}

Here's a solution for retaining the case (upper, lower or capitalized):

// Assumes $replace is already lowercase
function convertCase($find, $replace) {
  if (ctype_upper($find) === true)
    return strtoupper($replace);
  else if (ctype_upper($find[0]) === true)
    return ucfirst($replace);
  else
    return $replace;
}

$find = 'hello';
$replace = 'hi';

// Find the word in all cases that it occurs in
while (($pos = stripos($input, $find)) !== false) {
  // Extract the word in its current case
  $found = substr($input, $pos, strlen($find));

  // Replace all occurrences of this case
  $input = str_replace($found, convertCase($found, $replace), $input);
}

You could try the following function. Be aware that it will only work with ASCII strings, as it uses some of the useful properties of ASCII upper and lower case letters. However, it should be extremely fast:

function preserve_case($old, $new) {
    $mask = strtoupper($old) ^ $old;
    return strtoupper($new) | $mask .
        str_repeat(substr($mask, -1), strlen($new) - strlen($old) );
}

echo preserve_case('Upper', 'lowercase');
// Lowercase

echo preserve_case('HELLO', 'howdy');
// HOWDY

echo preserve_case('lower case', 'UPPER CASE');
// upper case

echo preserve_case('HELLO', "howd'y");
// HOWD'Y

This is my PHP version of the clever little perl function:

How do I substitute case insensitively on the LHS while preserving case on the RHS?