Dim regEx As New RegExp
With regEx
    .Global = True 'Matches whole string, not just first occurrence
    .IgnoreCase = True 'Matches upper or lowercase
    .MultiLine = True 'Checks each line in case cell has multiple lines
    .pattern = "^(d{2,})" 'Checks beginning of string for at least 2 digits
End With

'cell is set earlier in code not shown, refers to an Excel cell
regEx.replace(cell.Value, "XX BLOCK")

"1091 foo address"      --> "10XX BLOCK foo address"
"1016 foo 1010 address" --> "10XX BLOCK foo 1010 address"
"foo 1081 address"      --> "foo 1081 address"
"10 bar address"        --> "XX BLOCK bar address"
"8 baz address"         --> "8 baz address"

^(d*)d{2}b

^(d*)d{2}(?!d) ' no digit is allowed after the 2-digit sequence
^(d*)d{2}(?!S) ' a whitespace boundary