Sorry, but I am not familiar with using an equal sign to extract from a variable to another variable, and I am unable to find a reference to that syntax in my OMU documentation.
Maybe you can explain why you need to do that. Normally, I assign data to a variable in the pattern matching ($6 \\<*.source>) and then use the variable where needed, such as in the message text ("message text contains <source> blah, blah").
If your text is "Customer123:HOST-IPBX): Device from HOST-IPBX is DOWN.", then, no, the pattern will not match because is is looking for the text between "\" and ")".