erc-button-url-regexp

erc-button-url-regexp is a variable defined in `erc-button.el'.
Its value is

"\\(www\\.\\|\\(s?https?\\|ftp\\|file\\|gopher\\|news\\|telnet\\|wais\\|mailto\\):\\)\\(//[-a-zA-Z0-9_.]+:[0-9]*\\)?[-a-zA-Z0-9_=!?#$@~`%&*+\\/:;.,()]+[-a-zA-Z0-9_=#$@~`%&*+\\/()]"

Documentation:
Regular expression that matches URLs.

You can customize this variable.