Spacing underscore and tilde in URLs in the 856 field:

In February 1994, additional characters in the MARC character set were defined to both accommodate existing bibliographic needs and to align it with the ASCII and ANSEL character sets.  Both the spacing underscore ( _ ) and the spacing tilde ( ~ ) were added at the time because of the need in directory and file names for electronic resources.

Many systems have implemented these characters, however, for systems that have not implemented the spacing underscore and tilde, the following alternative characters may be used:

%5F  for spacing underscore

%7E  for spacing tilde