How to drop columns by name pattern in R?

Question

I have this dataframe:

state county city  region  mmatrix  X1 X2 X3    A1     A2     A3      B1     B2     B3      C1      C2      C3

  1      1     1            


        

Answer 1

Your code works like a charm if I apply it to a minimal example and just search for the string "A":

df <- data.frame(ID = 1:10,
                 A1 = rnorm(10),
                 A2 = rnorm(10),
                 B1 = letters[1:10],
                 B2 = letters[11:20])
df[, -grep("A", colnames(df))]

So your problem is more a regular expression problem, not how to drop columns. If I run your code, I get an error:

df[, -grep("\\3$", colnames(df))]
Error in grep("\\3$", colnames(df)) : 
  invalid regular expression '\3$', reason 'Invalid back reference'

Update: Why don't you just use this following expression?

df[, -grep("1$", colnames(df))]
   ID         A2 B2
1   1  2.0957940  k
2   2 -1.7177042  l
3   3 -0.0448357  m
4   4  1.2899925  n
5   5  0.7569659  o
6   6 -0.5048024  p
7   7  0.6929080  q
8   8 -0.5116399  r
9   9 -1.2621066  s
10 10  0.7664955  t