R REGEX Match - at least 1 lowercase letter, 1 number, and no special characters at 8 length

Question

I'm trying to create a grepl regex in R to match strings that:

1. Contain 1 or more lowercase letters
2. Contain 1 or more numbers
3. Only allow lowercase letters (a-z) or numbers, i.e. no spaces, no special characters, no other punctuation
4. The string must be exactly 8 characters long

However, my attempt so far doesn't yield any luck:

grepl("((?=.*[[:lower:]])(?=.*[[:digit:]])[[:alpha:]]{8})", x, perl=TRUE)

Any ideas where I'm going wrong?

Examples of inclusion cases would be: xxxxxxx8, 1234567x, ab12ef78

Examples of exclusion cases would be: x!3d5f78, x23456789, Ab123456

Answer 1

You're very close, you have the key concepts right (mainly forward lookahead). You could use this:

grepl("((?=.*[[:lower:]])(?=.*[[:digit:]])[[:lower:][:digit:]]{8})", x, perl=TRUE)

Personally, I don't find it much more readable to use named character classes, so I'd write it like this:

grepl("^(?=.*[a-z])(?=.*\\d)[a-z\\d]{8}$", x, perl=TRUE)

I also removed the outer parens (not necessary) and anchored the beginning & end.

Here are the results on your example inputs:

x <- c("xxxxxxx8", "1234567x", "ab12ef78", "x!3d5f78", "x23456789", "Ab123456")

grepl("^(?=.*[a-z])(?=.*\\d)[a-z\\d]{8}$", x, perl=TRUE)
# [1]  TRUE  TRUE  TRUE FALSE FALSE FALSE

Answer 2

You could also manage with very simple regex by breaking up your test:

grepl("[a-z]", x) & # Contain 1 or more lowercase letters
  grepl("\\d", x) & # Contain 1 or more numbers
  !grepl("[A-Z]|\\s|\\p{P}|\\p{S}", x, perl = TRUE) & # no upper, space, punctuation nor special char.
  nchar(x) == 8L # is 8 characters

[1]  TRUE  TRUE  TRUE FALSE FALSE FALSE