RegEx's that you can use to match @media rules are:

^\@media\s(.*?)\{$

^ = assert position at start of the string
\@ = matches the character @ literally
media = matches the characters media literally
\s = match any white space character [\r\n\t\f ]
.*? = matches any character (except newline)
{ = matches the character { literally
$ = assert position at end of the string
\@media\s(.*?)\{

\@ = matches the character @ literally
media = matches the characters media literally
\s = match any white space character [\r\n\t\f ]
.*? = matches any character (except newline)
{ = matches the character { literally
Also you can use the i to make the RegEx's case insensitive (i only ignores the case of [a-zA-Z])

Here are four possible ways of detecting @media rules using RegEx:

/^\@media\s(.*?)\{$/
/^\@media\s(.*?)\{$/i
/\@media\s(.*?)\{/
/\@media\s(.*?)\{/i