How to extract numeric part with "e" from a string

I have a string like "reflectance_scales=5.011129178e-05". I want to extract the part "5.011129178e-05" in numeric form.

I have tried:

gsub("[^0-9.]", "",  "reflectance_scales=5.011129178e-05")

but it extracts only 5.011129178 and not the e-05.