Extract numbers from string values of one column in df to another

Question

I would like to extract numbers from string values in one of the columns of my data frame and put them into another column. All the values in column "a" are unique. This code brings column "b" full of NA

df$b <- lapply(df$a,function(i){as.numeric(substr(df[i,"a"], 
regexpr("abc[0-9]+",df[i,"a"]) + 2, regexpr("abc[0-9]+",df[i,"a"]) + 
attr(regexpr("abc[0-9]+", df[i,"a"]), "match.length") -1))})

Answer 1

We can try

 library(stringr)
 as.numeric(unlist(str_extract_all(df$a, '\\d+')))