13.5 Use grep and regular expressions to retrieve columns by their names

Example of a data frame:

# Build data frame
df_regex <- data.frame(expression1=1:4, 
    expression2=2:5, 
    expression3=4:7, 
    annotation=LETTERS[1:4], 
    expression4=6:3, 
    average_expression=c(3.25, 3.75, 4.25, 4.75),
    stringsAsFactors=FALSE)

# Select column names that start with "expression"
grep(pattern="^expression", 
    x=colnames(df_regex))

## [1] 1 2 3 5

# Select columns from df_regex if their names start with "expression"
df_regex[, grep(pattern="^expression", colnames(df_regex))]

##   expression1 expression2 expression3 expression4
## 1           1           2           4           6
## 2           2           3           5           5
## 3           3           4           6           4
## 4           4           5           7           3