Imports System

Imports System.Collections.Generic

Public Module Module1

​

    Public Sub Main()

        Dim input As String = String.Join(Environment.NewLine, { _

                "@hello friend", _

                "@ hello friend", _

                "hey@hello @hello", _

                "@hello! hi @hello #hi ##hello"})

​

        Console.WriteLine("Mentions: {0}", String.Join(", ", FindTags("@"c, input)))

        Console.WriteLine("Hashtags: {0}", String.Join(", ", FindTags("#"c, input)))

        Console.ReadLine()

    End Sub

​

    Private Function FindTags(ByVal lead As Char, ByVal source As String) As String()

        Dim matches As List(Of String) = New List(Of String)

        Dim current_index As Integer = 0

​

        'Loop through all but the last character in the source

        For index As Integer = 0 To source.Length - 2

            'Reset the current index

            current_index = index

​

        'Check if the current character is a "@" or "#" and either we're starting at the beginning of the String or the last character was whitespace and then if the next character is a letter, digit, or end of the String

            If source(index) = lead AndAlso (index = 0 OrElse Char.IsWhiteSpace(source, index - 1)) AndAlso (Char.IsLetterOrDigit(source, index + 1) OrElse index + 1 = source.Length - 1) Then

                'Loop until the next character is no longer a letter or digit

                Do

                    current_index += 1

                Loop While current_index + 1 < source.Length AndAlso Char.IsLetterOrDigit(source, current_index + 1)

​

                'Check if we're at the end of the line or the next character is whitespace

                If current_index = source.Length - 1 OrElse Char.IsWhiteSpace(source, current_index + 1) Then

                    'Add the match to the collection

                    matches.Add(source.Substring(index, current_index + 1 - index))

                End If

            End If

        Next

​

        Return matches.ToArray()

    End Function

​

End Module