$re = '/(?<=ab|cd|ef|gh) *}} *{{ *(?=ab|cd|ef|gh)/m';
$str = '{{ab}}{{ cd }}{{ ef }} {{ gh }} {{ gh }}{{ghen web|url=tr.uc}}
machintrcd}} {{ab}} {{cd}}
{{ab}}{{ab}}{{ab}}{{ab}}{{ab}}{{ab}}
{{ab}} {{ab}} {{ab}} {{ab}} {{ab}} {{ab}}
machintruc {{ab}} {{cd}}
{{ab}} {{cd}} {{ef}} [https://truc.org truc]
{{mr}} Trucmuche';
$subst = "|";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php