regex - PHP regular expression -

what purpose of following code?

preg_replace( '@<(script|style)[^>]*?>.*?</\\1>@si', '', $string ); 

what kind of $string match expression?

why there character @?

that regular expression match <script>...</style> or <style>...</style> (x)html blocks in string , remove them. done prevent users inserting these (potentially harmful) tags data might echo user. if not removed, allow malicious users change site appearance, or insert javascript site rewrites page content; might force users visit other websites automatically , many other nasty things.

as @.... when defining regular expressions, traditionally enclosed slash example:

/regexphere/si 

the / around regular expression indicates boundaries , characters trailing second slash there flags regular expression engine behave way. in particular i means "case insensitive" , s means . in expression should match whitespace newlines , tabs. format inherited php perl , other unix utilities predate it.

other characters (like @ or | or %) can used replace / around regular expression though avoid unnecessary escaping when there lot of /s in pattern. example, it's easier , more readable write @http://@ /http:\/\//. in pattern makes easier not escape / in closing tag.