>Anyone knows a short and easy function to extract 7 digit string
>from a memo field ?

Look at using the Vbscript.RegExpn automation object, which supports regular expression parsing, something like:

*  Grab as many 7 digit strings as occur in this memo field instance
oRegExpn = CREATE('VBScript.RegExp')
oRegExpn.Global = .t.
oRegExpn.Pattern = "(\d{7})"
SCAN
   oSevenDigitStrings = oRegExpn.Execute(mMemoField)
   FOR EACH item IN oSevenDigitStrings
      ? item.value
   ENDFOR
ENDSCAN

WSH 2.0 must be installed on the target system to ensure that the VBScript.RegExp object will be available.