Implementations of regex functionality using a particular type of syntax is called a regex engine. There are many types of regex engines available. The most popular among them are: 1) PCRE (PHP) 2) JavaScript 3) Python 4) Golang Different regex engines support different types of syntax and the meaning of metacharacters may change depending upon the regex engine being used. Thus a regular expression that is considered valid under one regex engine may not be considered valid under another regex engine. Whenever you test a regex using a regex tester tool, you get the option to select the flavor under which you want to test your regular expression.