Look up the following string handling functions in the VBA help

Split, Mid, Left, InStr, InStrRev, Right.

Between them, they provide all the tools you need for this task.

I'm not sure if you are asking about how to parse filename or data, so
I'll take a stab at the filename.

Sub ParseFields2()
Dim MyString As String

MyString = "<DOC_ID>_<DOC_REV>_<DOC_PROD>_<DOC_NAME>.ext"

Field1$ = Mid(MyString, 2, 6)
Field2$ = Mid(MyString, 11, 7)
Field3$ = Mid(MyString, 21, 8)
Field4$ = Mid(MyString, 32, 8)

MsgBox (Field1$ & vbCr & Field2$ & vbCr & Field3$ & vbCr & Field4$)

End Sub

Or, this one is a little more dynamic.  If the field names change,
it'll still parse correctly. (Assuming the chevrons and underscores
don't change.)

Sub ParseFields()

Dim MyString As String
Dim MyArray() As String

MyString = "<DOC_ID>_<DOC_REV>_<DOC_PROD>_<DOC_NAME>.ext"
MyString = Mid(MyString, 2, InStr(MyString, ">.") - 2)
MyString = Replace(MyString, ">_<", ">=<")
MyArray = Split(MyString, ">=<")

MsgBox (MyArray(0) & vbCr & MyArray(1) & vbCr & MyArray(2) & vbCr &
MyArray(3))

End Sub