PHP Regexp replace all question marks before a slash

[svcghost]

Hey guys,

I've been struggling with this tricky REGEXP replace..

Basically, I want to replace all question marks with an @ symbol that appear BEFORE a slash. Only the ones that appear before the slash, leaving the ones that appear after the slash untouched.

For example:
Before

hey?how?are?you/i?am?fine

After

hey@how@are@you/i?am?fine

I'd reallllyyy appreciate some help.