Imports System.Text.RegularExpressions

Module PermutationSearch

    Public Sub Main()
        Dim input As String = "abcd"
        Dim length As Integer = 3
        Dim pattern As String = "*ab*"  ' This could be any pattern

        Dim permutations As New List(Of String)()
        GetPermutations("", input, length, permutations)

        ' Filter and output results based on user-defined pattern
        Dim regexPattern As String = WildcardToRegex(pattern)
        Dim result As New List(Of String)
        For Each s In permutations
            If Regex.IsMatch(s, regexPattern) Then
                result.Add(s)
                Debug.WriteLine(s)
            End If
        Next
    End Sub

    Private Sub GetPermutations(solution As String, possibleChars As String, Optional minStrLen As Integer = 1, Optional ByRef solutions As List(Of String) = Nothing)
        If solutions Is Nothing Then
            solutions = New List(Of String)()
        End If

        If solution.Length >= minStrLen Then
            solutions.Add(solution)
        End If

        For i As Integer = 0 To possibleChars.Length - 1
            Dim newSolution As String = solution & possibleChars(i)
            Dim newPossibleChars As String = possibleChars.Remove(i, 1) ' Remove the used character
            GetPermutations(newSolution, newPossibleChars, minStrLen, solutions)
        Next
    End Sub

    Private Function WildcardToRegex(pattern As String) As String
        ' Escape regex special characters and replace wildcard characters
        Dim regexPattern As String = Regex.Escape(pattern)
        regexPattern = regexPattern.Replace("\*", ".*") ' Replace * with .*
        regexPattern = regexPattern.Replace("\?", ".")   ' Replace ? with .
        Return "^" & regexPattern & "$" ' Ensure that the pattern matches the entire string
    End Function

End Module

''abc
''abcd
''abd
''abdc
''cab
''cabd
''cdab
''dab
''dabc
''dcab