<?php
 
$re11='/=(?=[A-Z])|(?<=[A-Z])=/';
$re12= '/=(?=[a-z0-9_\'-])|(?<=[a-z0-9_\'-])=/';
$re = '/=(?=[\w\'-])|(?<=[\w\'-])=/';
$str = 'I\'ve got a many web APP=. My app= is not working fast. It\'= slow app=';
 
 
echo "\n #### condition 1: all contexual upper or lowercase s \n";
$subst = 'S';
$result = preg_replace($re11,'S', $str);
$result = preg_replace($re12,'s', $result);
echo $result;
 
 
 
 
echo "\n ##### condition 2: all small case s \n";
$subst = 's';
$result = preg_replace($re, $subst, $str);
echo $result;
 
echo "\n ##### condition 3: all upper case S \n";
$subst = 'S';
$result = preg_replace($re, $subst, $str);
echo $result;
 
?>

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