[R] How to choose columns in data.frame by parts of columns' names?

[Guo Wei-Wei]

Dear all,

I have a data.frame which has names as following.
[1] "XG1"  "YG1"  "XEST"     "YEST"
[2] "XNOEMP1"  "XNOEMP2" "YNOEMP1"  "YNOEMP2"
[3] "XBUS10"   "XBUS10A" "XBUS10B"  "XBUS10C"
[4] "YBUS10"   "YBUS10A"  "YBUS10B"  "YBUS10C"
[5] "XOWNBUS"  "XSELFEST"  "YOWNBUS"  "YSELFEST"

Those columns have names beginning with "X" or "Y". Each "X" is paired
by a "Y", e.g. "XG1" and "YG1", but they are not in the order of "X Y
X Y ...". I want to combine "X*" and "Y*" like this:

data.new[,"G1"]  <- (data.old[,"XG1"] + endata.use[,"YG1"])/2

How to choose columns by parts of names? For example, I can pick out
XG1 and YG1 because they have the common part "G1".

Thank you.

Wei-Wei