Given:

dat1 <- setDT(data.frame(pat=c("A.C",".BC"),val=c(1,2)))
dat2 <- setDT(data.frame(q=c("ABC","AXC","XBC"),val2=c(10,11,12)))

I want the result to be:

dat3 <- setDT(data.frame(pat=c("A.C","A.C",".BC",".BC"),val=c(1,1,2,2),q=c("ABC","AXC","ABC","XBC"),val2=c(10,11,10,12)))
dat3
   pat val   x val2
1: A.C   1 ABC   10
2: A.C   1 AXC   11
3: .BC   2 ABC   10
4: .BC   2 XBC   12

In other words, a left join of dat1 to dat2 for each match of regex pattern pat to query string q
I was wondering if this is possible with the data.table merge syntax, i.e.

dat1[dat2, .(pat, val, q, val2), on= .(grepl(pat,q))] # this does not work

or any other data.table trick for efficiency. In reality df1 is hundreds of rows and df2 can be 10-100 thousand rows.