RegEx Problem using .NET

Question

I have a little problem on RegEx pattern in c#. Here's the rule below:

input: 1234567 expected output: 123/1234567

Rules:

1. Get the first three digit in the input. //123
2. Add /
3. Append the the original input. //123/1234567
4. The expected output should looks like this: 123/1234567

here's my regex pattern:

regex rx = new regex(@"((\w{1,3})(\w{1,7}))");

but the output is incorrect. 123/4567

Answer 1

It's not clear why you need a RegEx for this. Why not just do:

string x = "1234567";
string result = x.Substring(0, 3) + "/" + x;

Answer 2

Use positive look-ahead assertions, as they don't 'consume' characters in the current input stream, while still capturing input into groups:

Regex rx = new Regex(@"(?'group1'?=\w{1,3})(?'group2'?=\w{1,7})");

group1 should be 123, group2 should be 1234567.

Answer 3

I think this is what you're looking for:

string s = @"1234567";
s = Regex.Replace(s, @"(\w{3})(\w+)", @"$1/$1$2");

Instead of trying to match part of the string, then match the whole string, just match the whole thing in two capture groups and reuse the first one.

Answer 4

Another option is:

string s = Regex.Replace("1234567", @"^\w{3}", "$&/$&"););

That would capture 123 and replace it to 123/123, leaving the tail of 4567.

• ^\w{3} - Matches the first 3 characters.
• $& - replace with the whole match.

You could also do @"^(\w{3})", "$1/$1" if you are more comfortable with it; it is better known.