How to replace string between delimiters more than once by regular expression in php

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PHP

regular-expression

string

web

I want to replace dynamic string value between certain prefix and suffix delimiter for multiple match. By using preg_replace() method in php I can set value between delimiter with following pattern. but it only works for last match.
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preg_replace("#(.*)$delimit(.*?)$delimit(.*)#is", "$1$attr=$delimit$inject$delimit$3", $contents);
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Please help me, how can I achieve this. and how to handle unpredictable white space between delimiter

"********* {delimiter} {old string} {delimiter}************{delimiter} {old string} {delimiter} *****************"
to
"*********{delimiter} {new string} {delimiter}***********{delimiter} {new string} {delimiter}*****************"
Please help me.Thanks alot.