how to subset rows using regular expression patterns

hi netters,

i have a dataframe A with several columns(variables). the elements of
column M are character strings. so
A$M=c("ab","abc","bcd","ac","abcd","fg",....."fl").

i wanna extract all the rows where A$M match some regular expression
pattern.
for a simple example, let the pattern be just "ab", i wanna subset the
rows where A$M="ab" or "abc" or "abcd" or "abXX".........

i know i can write a loop,using some regular expression pattern
functions like grep row by row. but when A's size is pretty large,
it's inefficient. could anyone give me a hint about a faster code?

thanks a lot!