C# conditional regex replace and phone formatting

Question

Is there any way to do a Regex Replace conditionally? Here's an example.

Say you have a number that you know corresponds to a phone, with no formatting:

5553331234

Formatted, it would be

(555)333-1234

However, we don't have the number formatted and want to apply formatting. You can do an easy regex match and capture each of the 3 groups with something along the following:

(\d{3})(\d{3})(\d{4})

From there, you can do a simple regex replace

($2)$3-$4

However, what if you might have an extension? Maybe you have more than the 10 standard phone digits:

555333123456789

Where 56789 would be an extension. In that case, I could match it with

(\d{3})(\d{3})(\d{4})(\d*)

If I want to format it, but only include the X for extension if it exists, can I do that? For instance, I could make my replace format:

($2)$3-$4x$5

However, if I did that, the x would show up even when there is no extension. Is there a way purely using the regex to make that x show up conditionally? Essentially it would be "if $5" exists or "if $5.Length() > 0".

As a workaround, I could include a named group in that regex match where (\d*) above becomes

(?<Conditional>\d*)

In the code, I could then assert the length of the group named "Conditional". However, I'd like to have a purely regex solution that does not require anything custom like that.

Lucas Trzesniewski · Accepted Answer · 2016-10-27 19:10:53Z

2

Take a look at Substitutions in Regular Expressions - there's no replacement pattern that would allow you to write a conditional. Other libraries, like boost::regex do provide such facilities, but you don't get that in .NET.

What you do get though is the ability to use a callback to supply the replacement. Just use a lambda:

Regex.Replace(
    "555333123456789",
    @"(\d{3})(\d{3})(\d{4})(\d+)?",
    m => $"({m.Groups[1].Value}){m.Groups[2].Value}-{m.Groups[3].Value}{(m.Groups[4].Success ? $"x{m.Groups[4].Value}" : "")}"
)

Yes, I know that's not a pure regex solution, but it does its job. Remember that substitution is not really a regex operation, it's rather a post-matching step which does involve the result of a match but does not involve the regex engine itself.

answered Oct 27, 2016 at 19:10

Lucas Trzesniewski

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juharr · Accepted Answer · 2016-10-27 19:10:17Z

1

You can use the overload of Regex.Replace that takes a MatchEvaluator which will allow you to do conditional logic based on the matched groups.

var phone = "5553331234";
Regex phoneRegex = new Regex(@"(\d{3})(\d{3})(\d{4})(\d*)");
var formatted = phoneRegex.Replace(
    phone, 
    m => string.Format(
        "({0}){1}-{2}{3}{4}", 
        m.Groups[1].Value, 
        m.Groups[2].Value, 
        m.Groups[3].Value, 
        m.Groups[4].Value.Length > 0 ? "x" : string.Empty, 
        m.Groups[4].Value));
Console.WriteLine(formatted);

answered Oct 27, 2016 at 19:10

juharr

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