Analyse format of alpha-numeric string - regex

Question

I'm trying to write a function that takes in a string, parses it, and returns another string that summarizes the number of consecutive alpha or numeric characters in the original string.
For example, the string 999aa45bbx would return 3N2A2N3A,
i.e.
3 numbers,
followed by 2 alpha,
by 2 numbers,
by 3 alpha.
I'm using the function to analyze formats of insurance policy ID numbers. So far, I've found solutions online that extract either alpha or numeric characters, but nothing that describes the format or order in which these characters exist in the original string.
Can anyone help?

Answer 1

A regexp like this will do the job
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Insert Module
copy and paste the code below
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then you can use the function (which also detects invalid strings) within Excel, ie in B1
=AlphaNumeric(A1)
Function AlphaNumeric(strIn As String) As String
Dim objRegex As Object
Dim objRegMC As Object
Dim objRegM As Object
Dim strOut As String
Set objRegex = CreateObject("vbscript.regexp")
With objRegex
.Global = True
.ignorecase = True
.Pattern = "[^\w]"
If .test(strIn) Then
AlphaNumeric = "One or more characters is invalid"
Else
.Pattern = "(\d+|[a-z]+)"
Set objRegMC = .Execute(strIn)
For Each objRegM In objRegMC
strOut = strOut & (objRegM.Length & IIf(IsNumeric(objRegM), "N", "A"))
Next
AlphaNumeric = strOut
End If
End With
End Function

Answer 2

Old school, looping through all characters in the string:
Function IdentifyCharacterSequences(s As String) As String
Dim i As Long
Dim charCounter As Long
Dim currentCharType As String
Dim sOut As String
sOut = ""
charCounter = 1
currentCharType = CharType(Mid(s, 1, 1))
For i = 2 To Len(s)
If (Not CharType(Mid(s, i, 1)) = currentCharType) Or (i = Len(s)) Then
sOut = sOut & charCounter & currentCharType
currentCharType = CharType(Mid(s, i, 1))
charCounter = 1
Else
charCounter = charCounter + 1
End If
Next i
IdentifyCharacterSequences = sOut
End Function
This uses the following helper function. Note that non-alphanumeric characters are identified using the letter "X". You can easily modify this to suit your purposes.
Function CharType(s As String) As String
If s Like "[A-z]" Then
CharType = "A"
ElseIf s Like "[0-9]" Then
CharType = "N"
Else
CharType = "X"
'Or raise an error if non-alphanumerical chars are unacceptable.
End If
End Function
Usage example: