With the really good online tool RegExr can be found easily matching Regular Expressions, which have been created by other users, as well as develop their own regular expressions.
With the PHP Regular Expression Tester the results can be very good test online:
Bsp: regular expression test for 3-digit numbers (not perfect, Leading zeros are not ignored).
Example 2: Email-Addresses validate glands
On the Regexr page I have the following regular expression found:
This expression must be converted for use in PHP:
Thus, man can then PHP Regular Expression Tester perform the test: