Question

I need a regex to match a string that:

• has only digits 0-9 and spaces
• all digits must be same
• should have at-least 2 digits
• should start and end with digits

Matches:

11
11111
1  1 1 1 1
1  1
11 1 1 1 1 1
1           1
1    1      1

No matches:

1             has only one digit
11111         has space at the end
 11111        has space at beginning
12            digits are different
11:           has other character

I know regex for each of my requirement. That way I’ll use 4 regex
tests. Can we do it in one regex?

Answer 1

Yes it can be done in one regex:

^(\d)(?:\1| )*\1$

Rubular link

Explanation:

^      - Start anchor
(      - Start parenthesis for capturing
 \d    - A digit
)      - End parenthesis for capturing
(?:    - Start parenthesis for grouping only
\1     - Back reference referring to the digit capture before
|      - Or
       - A literal space
)      - End grouping parenthesis
*      - zero or more of previous match
\1     - The digit captured before
$      - End anchor