Original post from Jimmy Rishe: http://www.infopathdev.com/forums/p/9479/33529.aspx

I needed to get the display name for the logged in user from active directory to populate a text box in an InfoPath form and went searching Google for an answer. Saddly to say, I fould some really complicated solutions that I didn't even want to attempt to deploy.

Luckily I found this article from jabit on CodeProject and it saved me a lot of (additional ) headache.
http://www.codeproject.com/KB/dotnet/UserName.aspx

For myself, I created two properties in the InfoPath code behind file and used it like this (I also had to add a reference to System.DirectoryServices.AccountManagement; you may too):

Imports System.DirectoryServices.AccountManagement
.
.
.
        Private GroupRequestor As String = "/my:myFields/my:groupRequestDetails"
        Private requestSubmittedBy As String = GroupRequestor & "/my:requestSubmittedBy"
        Private requestSubmittedByLogin As String = GroupRequestor & "/my:requestSubmittedByLogin"

        Private ReadOnly Property Fullname() As String
            Get
                Dim displayName As String
                Try
                    displayName = UserPrincipal.Current.DisplayName
                Catch ex As Exception
                    displayName = System.Environment.UserName
                End Try

                Return displayName
            End Get
        End Property

        Private ReadOnly Property Username() As String
            Get
                Return Application.User.LoginName
            End Get
        End Property

        Private Sub SetValue(ByVal node As String, ByVal value As String)
            Dim root As XPathNavigator = MainDataSource.CreateNavigator()

            root.SelectSingleNode(node, NamespaceManager).SetValue(value)
        End Sub

        Private Function GetValue(ByVal node As String) As String
            Dim root As XPathNavigator = MainDataSource.CreateNavigator()

            Return root.SelectSingleNode(node, NamespaceManager).Value
        End Function

        Public Sub FormEvents_Loading(ByVal sender As Object, ByVal e As LoadingEventArgs)
            If GetValue(requestSubmittedBy) = String.Empty Then
                SetValue(requestSubmittedBy, Me.Fullname)
                SetValue(requestSubmittedByLogin, Me.Username)
            End If
        End Sub

Here's my custom pattern; it forces the user to enter the cents as well.

$?((\d{1,3}(,\d{3})*)|\d*)(\.\d{2})

Matches:
$1,000.00
1,000.00
1000.00
$1000.00