# [Haskell], <s>50</s> 48 bytes
<!-- language-all: lang-hs -->
f[]=1
f('[':a:b:c:s)=length[a..c]*f s
f(a:s)=f s
[Try it online!][TIO-k75zqm0x]
[Haskell]: https://www.haskell.org/
[TIO-k75zqm0x]: https://tio.run/##nY1BDoJADEX3nqJOjCixBJZOxMRzNF10gAEiTIiw8vLjMIQLuHl9/zdNO5nfzTB4b4nL4mAvCSVatNGVnq/l0Lh26UiyrOLUwhz2svZB/Si9gxKmT@8WOMEoE1ggpW6gZAWlmHOUHO8cEZPgl2t6Bcb4wOcmRzxvUoeVGBunbWNFyPuxMBk0XNUNIeL@oOA/odj/AA "Haskell – Try It Online"