PHP preg_replace() is a perl style regular expression function. It replaces the specific patterns inside a string using regular expression. The regular expression pattern must be put inside / /. preg_replace() function is much slower than other string functions such as str_replace() which do not support regular expressions.

<?PHP
$str="2000-08-12T03:54:54Z";
$str2=preg_replace("/\-/","/",$str);
echo "$str2"; //2000/08/12T03:54:54Z
?>

preg_replace() function can match different patterns using | symbol.

<?PHP
$str="2000-08-12T03:54:54Z";
$str2=preg_replace("/\-|:/"," ",$str);
echo "$str2"; //2000 08 12T03 54 54Z
?>

When matching multiple patterns using | symbol, preg_replace() function replace the 1st occurrence first, followed by the 2nd occurrence and so on.

<?PHP
$str="what time is the best time when there is no time at all";
$str2=preg_replace("/ time | is |when/","",$str);
//" is " is not replaced since the space has been replaced by " time "
echo "$str2"; //whatis the best therenoat all
$str3=preg_replace("/ time | is | when/","",$str);
//" is " and " when" are not replaced since the spaces
// have been replaced by " time "
echo "$str3"; //whatis the bestwhen therenoat all
?>

You may use () to save reserved patterns. \\1 represents the 1st occurrence of replacement, \\2 the 2nd and so on.

<?PHP
$str="2000-08-12T03:54:54Z";
$str2=preg_replace("/\-(\d+)\-(\d+)T/","/\\2/\\1T",$str);
echo "$str2"; //2000/12/08T03:54:54Z
?>