message-cite-prefix-regexp

message-cite-prefix-regexp is a variable defined in `message.el'.
Its value is

"\\([ 	]*\\(\\w\\|[_.]\\)+>+\\|[ 	]*[]>|]\\)+"

Documentation:
*Regexp matching the longest possible citation prefix on a line.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.