Truncate string after certain number of occurrences of a substring

Question

Let's say I have a string variable:

$string = "1 2 3 1 2 3 1 2 3 1 2 3";

I want to cut off the end of this string starting at the fourth occurrence of the substring "2", so $string is now equal to this:
1 2 3 1 2 3 1 2 3 1.

Effectively cutting off the fourth occurrence of "2" and everything after it.

How would one go about doing this? I know how to count the number of occurrences with substr_count($string,"2");, but I haven't found anything else searching online.

Answer 1

To find the position of the fourth 2 you could start with an offset of 0 and recursively call $offset = strpos($str, '2', $offset) + 1 while keeping track of how many 2's you've matched so far. Once you reach 4, you just can just use substr().

Of course, the above logic doesn't account for false returns or not enough 2's, I'll leave that to you.

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You could also use preg_match_all with the PREG_OFFSET_CAPTURE flag to avoid doing the recursion yourself.

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Another option, expanding on @matt idea:

implode('2', array_slice(explode('2', $string, 5), 0, -1));

Answer 2

May be this would work for you:

$str = "1 2 3 1 2 3 1 2 3 1 2 3"; // initial value
preg_match("#((.*)2){0,4}(.*)#",$str, $m);
//var_dump($m);
$str = $m[2]; // last value

Answer 3

This code snippet should do it:

implode($needle, array_slice(explode($needle, $string), 0, $limit));

Answer 4

$string = explode( "2", $string, 5 );
$string = array_slice( $string, 0, 4 );
$string = implode( "2", $string );

See it here in action: http://codepad.viper-7.com/GM795F

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To add some confusion (as people are won't to do), you can turn this into a one-liner:

implode( "2", array_slice( explode( "2", $string, 5 ), 0, 4 ) );

See it here in action: http://codepad.viper-7.com/mgek8Z

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For a more sane approach, drop it into a function:

function truncateByOccurence ($haystack, $needle,  $limit) {
    $haystack = explode( $needle, $haystack, $limit + 1 );
    $haystack = array_slice( $haystack, 0, $limit );
    return implode( $needle, $haystack );
}

See it here in action: http://codepad.viper-7.com/76C9VE

Answer 5

How about something simple like

$newString = explode('2',$string);

and then loop through the array as many times as occurrences you need:

$finalString = null;
for($i=0:$i<2;$i++){
    $finalString .= 2 . $newString[$i];
}

echo $finalString;

Answer 6

Match zero or more non-2 characters followed by a 2 -- repeated 4 times.

Restart the fullstring match before every matched 2 with \K.

After successfully matching four 2's, match the remaining characters and replace the matched characters with an empty string.

$string = "1 2 3 1 2 3 1 2 3 1 2 3";

echo preg_replace(
         '~([^2]*\K2){4}.*~',
         '',
         $string
     );

I have similar pattern advice at this answer which splits a string on every n characters.