How do I select variables in an R dataframe whose names contain a particular string?

Question

Two examples would be very helpful for me.

How would I select: 1) variables whose names start with b or B (i.e. case-insensitive) or 2) variables whose names contain

Answer 1

If you just want the variable names:

grep("^[Bb]", names(df), value=TRUE)

grep("3", names(df), value=TRUE)

If you are wanting to select those columns, then either

df[,grep("^[Bb]", names(df), value=TRUE)]
df[,grep("^[Bb]", names(df))]

The first uses selecting by name, the second uses selecting by a set of column numbers.