Regex force length of specific regex [closed]

Question

I'm using R and need a regex for

a block of N characters starting with zero or more whitespaces and continuing with one or more digits afterwards

For N = 9 here are

examples of valid strings

• 123456789
• kfasdf 3456789asdf
• a 1

and examples of invalid strings

• 12345 789
• 1 9
• a 678a

Answer 1

Another option is to match 8 times either a digit OR a space not preceded by a digit and then match a digit at the end.

(?<![\d\h])(?>\d|(?<!\d)\h){8}\d

In parts

• (?<![\d\h]) Negative lookbehind, assert what is on the left is not a horizontal whitespace char or digit
• (?> Atomic group (no backtracking)
  • \d Match a digit
  • | Or
  • \h(?<!\d\h) Match a horizontal whitespace char asserting that it is not preceded by a digit
• ){8} Close the group and repeat 8 times
• \d Match the last digit

Regex demo | R demo

Example code, using perl=TRUE

x <- "123456789
kfasdf  3456789asdf
a        1

12345 789
1       9
a     678a"
    regmatches(x, gregexpr("(?<![\\d\\h])(?>\\d|(?<!\\d)\\h){8}\\d", x, perl=TRUE))

Output

[[1]]
[1] "123456789" "  3456789" "        1"

If there can not be a digit present after matching the last 9th digit, you could end the pattern with a negative lookahead asserting not a digit.

(?<![\d\h])(?>\d|(?<!\d)\h){8}\d(?!\d)

Regex demo

If there can not be any digits on any side:

 (?<!\d)(?>\d|(?<!\d)\h){8}\d(?!\d)

Regex demo

Answer 2

Using string s from @d.b's answer.

Extract optional whitespace followed by numbers.

library(stringr)
str_extract(s, '(\\s+)?\\d+')
#[1] "123456789" "  3456789" "        1" "12345"     "1"         "     678" 

Check their length using nchar.

nchar(str_extract(s, '(\\s+)?\\d+')) == 9
#[1]  TRUE  TRUE  TRUE FALSE FALSE FALSE

Using the same logic in base R function.

nchar(regmatches(s, regexpr('(\\s+)?\\d+', s))) == 9
#[1]  TRUE  TRUE  TRUE FALSE FALSE FALSE

---

If there could be multiple such instances we can use str_extract_all :

sapply(str_extract_all(s, '(\\s+)?\\d+'), function(x) any(nchar(x) == 9))