city       ([FY](?=[^P]))|(W(?=[^MP]))
chymotrypsin low specificity        ([FLY](?=[^P]))|(W(?=[^MP]))|(M(?=[^PY]))|(H(?=[^DMPW]))
clostripain                         R
cnbr                                M
enterokinase                        (?&lt;=[DE]{3})K
factor xa                           (?&lt;=[AFGILTVM][DE]G)R
formic acid                         D
glutamyl endopeptidase              E
granzyme b                          (?&lt;=IEP)D
hydroxylamine                       N(?=G)
iodosobenzoic acid    