       return [getArgValue cell for cell in cells]
 
     # matrix
-    if /^\w\d{1,2}:\w\d{1,2}$/.exec expr
+    if /^[A-Z]\d{1,2}:[A-Z]\d{1,2}$/.exec expr
       refs = expr.split(':')
       colStart = refs[0][0].toUpperCase().charCodeAt(0) - 65 # A turns into 0
       colEnd   = refs[1][0].toUpperCase().charCodeAt(0) - 65


    # arithmetic (or number)
    try
      return eval expr.replace /(\w\d{1,2})/g, (addr) =>
        getCalcResultAt(addr) or 0
    catch e
      return '#VALUE'

    expr = expr.trim()

    # cell
    if /^[A-Z]\d{1,2}$/.exec expr
      return getCalcResultAt expr

    # list of cells
