Replace last occurrence of a string in a string

Another 1-liner but without preg:

$subject = 'bourbon, scotch, beer';
$search = ',';
$replace = ', and';

echo strrev(implode(strrev($replace), explode(strrev($search), strrev($subject), 2))); //output: bourbon, scotch, and beer

$string = 'this is my world, not my world';
$find = 'world';
$replace = 'farm';
$result = preg_replace(strrev("/$find/"),strrev($replace),strrev($string),1);
echo strrev($result); //output: this is my world, not my farm

The following rather compact solution uses the PCRE positive lookahead assertion to match the last occurrence of the substring of interest, that is, an occurrence of the substring which is not followed by any other occurrences of the same substring. Thus the example replaces the last 'fox' with 'dog'.

$string = 'The quick brown fox, fox, fox jumps over the lazy fox!!!';
echo preg_replace('/(fox(?!.*fox))/', 'dog', $string);

OUTPUT: 

The quick brown fox, fox, fox jumps over the lazy dog!!!

You could do this:

$str = 'Hello world';
$str = rtrim($str, 'world') . 'John';

Result is 'Hello John';

This is an ancient question, but why is everyone overlooking the simplest regexp-based solution? Normal regexp quantifiers are greedy, people! If you want to find the last instance of a pattern, just stick .* in front of it. Here's how:

$text = "The quick brown fox, fox, fox, fox, jumps over etc.";
$fixed = preg_replace("((.*)fox)", "$1DUCK", $text);
print($fixed);

This will replace the last instance of "fox" to "DUCK", like it's supposed to, and print:

The quick brown fox, fox, fox, DUCK, jumps over etc.

This will also work:

function str_lreplace($search, $replace, $subject)
{
    return preg_replace('~(.*)' . preg_quote($search, '~') . '(.*?)~', '$1' . $replace . '$2', $subject, 1);
}

UPDATE Slightly more concise version (http://ideone.com/B8i4o):

function str_lreplace($search, $replace, $subject)
{
    return preg_replace('~(.*)' . preg_quote($search, '~') . '~', '$1' . $replace, $subject, 1);
}

You can use strrpos() to find last match.

$string = "picture_0007_value";
$findChar =strrpos($string,"_");

$string[$findChar]=".";

echo $string;

Output : picture_0007.value

$string = "picture_0007_value";
$findChar =strrpos($string,"_");
if($findChar !== FALSE) {
  $string[$findChar]=".";
}

echo $string;

Apart from the mistakes in the code, Faruk Unal has the best anwser. One function does the trick.

Shorthand for accepted answer

function str_lreplace($search, $replace, $subject){ 
    return is_numeric($pos=strrpos($subject,$search))?
    substr_replace($subject,$replace,$pos,strlen($search)):$subject;
}

A short version:

$NewString = substr_replace($String,$Replacement,strrpos($String,$Replace),strlen($Replace));

While using regex is typically less performant than non-regex techniques, I do appreciate the control and flexibility that it affords.

In my snippet, I will set the pattern to be case-insensitive (\i, although my sample input will not challenge this rule) and include word boundaries (\b, although they were not explicitly called for).

I am also going to use the \K metacharacter to reset the fullstring match so that no capture groups / backreferences are needed.

Code: (Demo)

$search = 'The';
$replace = 'A';
$subject = "The Quick Brown Fox Jumps Over The Lazy Dog's Thermos!";

echo preg_replace(
         '/.*\K\b' . preg_quote($search, '/') . '\b/i',
         $replace,
         $subject
     );

Output:

  The Quick Brown Fox Jumps Over A Lazy Dog's Thermos!
# ^^^                            ^            ^^^
# not replaced                replaced        not replaced

Without word boundaries: (Demo)

echo preg_replace(
         '/.*\K' . preg_quote($search, '/') . '/i',
         $replace,
         $subject
     );

Output:

  The Quick Brown Fox Jumps Over The Lazy Dog's Armos!
# ^^^                            ^^^            ^
# not replaced              not replaced        replaced

Below corrected zack's code (https://stackoverflow.com/a/11144999/9996503). Be careful with regular expressions! Consider this:

$string = 'Round brackets (parentheses) "()", square brackets "()"';
$find = '()';
$replace = '[]';
// Zack's code:
$result = preg_replace(strrev("/$find/"),strrev($replace),strrev($string),1);
var_dump($result); // Output: NULL
// Correct code:
$result = strrev(preg_replace('/'.preg_quote(strrev($find)).'/', strrev($replace), strrev($string), 1));
echo $result; //Output: Round brackets (parentheses) "()", square brackets "[]"

Use the "$" on a reg expression to match the end of the string

$string = 'The quick brown fox jumps over the lazy fox';
echo preg_replace('/fox$/', 'dog', $string);

//output
'The quick brown fox jumps over the lazy dog'

For the interested: I've written a function that utilises preg_match so that you're able to replace from right hand side using regex.

function preg_rreplace($search, $replace, $subject) {
    preg_match_all($search, $subject, $matches, PREG_SET_ORDER);
    $lastMatch = end($matches);

    if ($lastMatch && false !== $pos = strrpos($subject, $lastMatchedStr = $lastMatch[0])) {
        $subject = substr_replace($subject, $replace, $pos, strlen($lastMatchedStr));
    }

    return $subject;
}

Or as a shorthand combination/implementation of both options:

function str_rreplace($search, $replace, $subject) {
    return (false !== $pos = strrpos($subject, $search)) ?
        substr_replace($subject, $replace, $pos, strlen($search)) : $subject;
}
function preg_rreplace($search, $replace, $subject) {
    preg_match_all($search, $subject, $matches, PREG_SET_ORDER);
    return ($lastMatch = end($matches)) ? str_rreplace($lastMatch[0], $replace, $subject) : $subject;
}

based on https://stackoverflow.com/a/3835653/3017716 and https://stackoverflow.com/a/23343396/3017716