[R] How to subset() from data frame using specific rows

[Rich Shepard]

   I have a data frame called chemdata with this structure:

> str(chemdata)
'data.frame':	14886 obs. of  4 variables:
  $ site    : Factor w/ 148 levels "BC-0.5","BC-1",..: 104 145 126 115 114 128 124 2 3 3 ...
  $ sampdate: Date, format: "1996-12-27" "1996-08-22" ...
  $ param   : Factor w/ 8 levels "As","Ca","Cl",..: 1 1 1 1 1 1 1 1 1 1 ...
  $ quant   : num  0.06 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 ...

   I've looked in the R Cookbook and Dalgaard's intro book without finding a
way to use wildcards (e.g., like "BC-*") or explicitly witing each site ID
when subdsetting a data frame..

   I need to create subsets (as data frames) based on sites, but including
all sites on each stream. For example, using the initial site factor shown
above, I want a subset containing all data for sites "BC-0.5", "BC-1".
"BC-2", "BC-3", "BC-4", "BC-5", and "BC-6".

Pointers appreciated,

Rich