Question

I need to write a regular expression that will be correct for telephone number with some rules:

1) max length 13 symbols
2) it should start from '+'
3) it should contain sonly numbers [0-9]

So, for example, it should be like this:

+447289347598342745

Answer 1

Regex re = new Regex(@"^\+\d{1,13}$");

• ^\+ → the string must start with +. Note that + must be escaped because it has special meaning otherwise.
• \d → equivalent to the character class [0-9]
• {1,13} → there must be at least one and no more than 13 occurences of the digit characters

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In .NET, you should either use [0-9] instead of \d, or else you should specify RegexOptions.ECMAScript. Otherwise, the regex will match a string like +୧٢३੪૫ (which contains Unicode digits 1-5 in various scripts). – Michael Liu

So either use:

Regex re = new Regex(@"^\+[0-9]{1,13}$");
// or
Regex re = new Regex(@"^\+\d{1,13}$", RegexOptions.ECMAScript);