Regular expressions are widely used in programming and intensions of regular expressions are to manipulate complex strings in terms of replacing parts or all of them, finding particular pattern, etc... .

PHP uses the POSIX extended regular expressions as defined by POSIX 1003.2 or a regular expression library is compatible with PERL called PCRE which consists a set of functions having the same syntax as PERL 5. 

You can learn more about REGEX (regular expressions) on these two links:

 

›› go to examples ››