[R] how to subset rows using regular expression patterns

[zhihua li]

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!