Find recursively all files whose content match a specific regular expression

I like to search all PHP files and find a particular string that is identified by a regular expression. The regular expressions that I use to find the string is: \$[a-zA-Z0-9]{5,8}\s\=\s.{30,50}\;\$[a-zA-Z0-9]{5,8}\s\=\s[a-zA-Z0-9]{5}\(\) I tried to use: grep -r "\$[a-zA-Z0-9]{5,8}\s\=\s.{30,50}\;\$[a-zA-Z0-9]{5,8}\s\=\s[a-zA-Z0-9]{5}\(\)" *.php but this does not seem to work. find . -name '*.php' -regex '\$[a-zA-Z0-9]{5,8}\s\=\s.{30,50}\;\$[a-zA-Z0-9]{5,8}\s\=\s[a-zA-Z0-9]{5}\(\)' -print Does not work either. I need is to search a path and all subdirectories for PHP files that contain a string identified by the regular expression stated above. What is the best way to accomplish this? For your information this is a string similar to the ones I try to find: As you probably realize, this is a malware code. So this string is similar but different on each file. However the regular expression code does a good job finding all files if it contains a similar content somewhere in the file. Before, I had downloaded all files to my windows PC and then used EMEditor to search by regular expression. This works fine on the PC, but for this I need to download everything and it would be nice to be able to search direct on Linux command prompt. Any tip would be very much appreciated.