Given a string like "abcdbcfbc" , I want all the occurrences of 'b' to be interchanged with their previous character.
The output should be: "bacbdcbfc"

I've come up with a temporary solution:

function punjabi_alternate($string) {
    $len = mb_strlen($string);
    $sploded = array(); 
    while($len-- > 0) {
        $sploded[$len] = mb_substr($string, $len, 1); 
    $len = mb_strlen($string);
    for ($i=0; $i < $len; $i++) {
        if( strcmp($sploded[$i], "b") == 0){
            $temp = $sploded[$i];
            $sploded[$i] = $sploded[$i-1];
            $sploded[$i-1] = $temp;
    return join('', $sploded);

Here I'm using mb_substr as my string contains unicode characters. The b is just an example. In my production version I'm using this instead of "b"


It is working as expected. I'd appreciate if there could be a cleaner way to do it. Also the strings passed to the function can contain as many as 10,000 - 20,000 characters. So, I'd prefer a preg_replace function with regex to do the same.

  • \$\begingroup\$ Related Stack Overflow question. It looks like you are asking for a regex-based solution rather than a review of the existing code, so I don't believe that Code Review is the appropriate place to ask this question. \$\endgroup\$ – 200_success Apr 5 '17 at 16:58
  • 1
    \$\begingroup\$ regex based solution was a secondary requirement. I just wanted to optimize the code for usage with large strings. \$\endgroup\$ – JiteshPubreja Apr 5 '17 at 23:22

You want to do a string replacement operation, so use preg_replace(). The key is to use backreferences in the replacement. Also, since you seem to be interested in processing Unicode, be sure to use the u flag for the regex.

If, as you say, 'b' is just an example, then you can make it a default parameter.

function punjabi_alternate($string, $char='b') {
    return preg_replace("/(.)($char)/u", '\2\1', $string);
  • \$\begingroup\$ Thanks a lot. It works. before i was trying the similar expression with . (Dot) and $char in single parenthesis . It was giving me single string containing both elements. It never occured to me to use both characters with different parentheses. \$\endgroup\$ – JiteshPubreja Apr 6 '17 at 8:32

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