Regex to match 7 same digits in a number regardless of position

Question

I want to match an 8 digit number. Currently, I have the following regex but It is failing in some cases.

(\d+)\1{6}

It matches only when a number is different at the end such as 44444445 or 54444444. However, I am looking to match cases where at least 7 digits are the same regardless of their position. It is failing in cases like

44454444
44544444
44444544

What modification is needed here?

why would you want to do that by regex. Performance would be terrible because it'll need to backtrack a lot — phuclv, Aug 03 '21 at 01:42

score 1 · Answer 1 · answered Aug 03 '21 at 01:54

You can sort the digits before matching

string input = "44444445 54444444 44454444 44544444 44444544";
string[] numbers = input.Split(' ');
foreach (var number in numbers)
{
    number = String.Concat(str.OrderBy(c => c));
    if (Regex.IsMatch(number, @"(\d+)\1{6}"))
        // do something
}

Still not a good idea to use regex for this though

score 1 · Accepted Answer · answered Aug 03 '21 at 02:23

It's probably a bad idea to use this in a performance-sensitive location, but you can use a capture reference to achieve this.

The Regex you need is as follows:

(\d)(?:.*?\1){6}

Breaking it down:

(\d) Capture group of any single digit
.*? means match any character, zero or more times, lazily
\1 means match the first capture group
We enclose that in a non-capturing group {?:
And add a quantifier {6} to match six times

score 0 · Answer 3 · answered Aug 03 '21 at 08:50

The pattern that you tried (\d+)\1{6} matches 6 of the same digits in a row. If you want to stretch the match over multiple same digits, you have to match optional digits in between.

Note that in .NET \d matches more digits than 0-9 only.

If you want to match only digits 0-9 using C# without matching other characters in between the digits:

([0-9])(?:[0-9]*?\1){6}

The pattern matches:

([0-9]) Capture group 1
(?: Non capture group
- [0-9]*?\1 Match optional digits 0-9 and a backreference to group 1
){6} Close non capture group and repeat 6 times

See a .NET Regex demo

If you want to match only 8 digits, you can use a positive lookahead (?= to assert 8 digits and word boundaries \b

\b(?=\d{8}\b)[0-9]*([0-9])(?:[0-9]*?\1){6}\d*\b

See another .NET Regex demo

Regex to match 7 same digits in a number regardless of position

3 Answers3