In Jabref, there is a menu bar entry to cleanup up bibtex entries.
How about the following regex to clean up the mrnumber from Math Scinet?
Replace (case insensitively)
(^\smrnumber\s\=\s{\d+).$
with
\1},
By this search and replace, the entry becomes a single number that can then easily be referenced online by
\providecommand{\mr}[1]{MR~\href{http://www.ams.org/mathscinet-getitem?mr=#1}{#1}}
in your bst file.
Instead, for example when obtaining the bibtex entry from Math Scinet itself, it often comes as a list of various, equivalent references to the entry on Math Scinet.
Ticket moved from /p/jabref/patches/209/
Could you please clarify what you are trying to do here? I can't see what kind of mrnumber the regex should match. Normally the bibtex export of MathSciNet already includes the information in the format mrnumber = 3537908.