Matt Purdy
Feb 11, 2021
(2 votes)

The versatility of using Regex in custom validation attributes.

Hello there,

To kick of my Episerver blogging activities I have decided to begin with the insight of a powerful combination that can be used to perform strict strongly typed server-side validation.

This example showcases the power of Regex to perform required phone number field validation, which, in this instance was all numeric except for specific characters reserved for area codes.

After creating a validation Regex pattern object, you then use the Regex isMatch method to validate your field data, thus getting your result.

Validation Attribute

public class PhoneNumberValidationAttribute : ValidationAttribute
        private readonly int _charLength;

        public PhoneNumberValidationAttribute(int charLength)
            this._charLength = charLength;

        public override bool IsValid(object value)
            var inputtedVal = value as string;

            var validator = new Regex(@"^[0123456789()+-]+$");

            var isValidInput = inputtedVal != null && (validator.IsMatch(inputtedVal) && inputtedVal.Length <= _charLength);

            return isValidInput;

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
            var result = base.IsValid(value, validationContext);

            if (!string.IsNullOrWhiteSpace(result?.ErrorMessage))
                result.ErrorMessage = $"Phone number must contain only numbers with the exception of the area code.";

            return result;

Further improvements would be more complex pattern matching or enhanced error message overloading.

Model property decoration

public virtual string TelephoneNumber { get; set; }

Whilst writing this post I came to learn of certain alternative data type attributes available within .NET (System.ComponentModel.DataAnnotations) that would fit most use cases. However, this approach is a still a desirable one when chasing a certain level of autonomy.

Feb 11, 2021


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