Extract a string before a string using regex [duplicate]

I am trying to write a regular expression which returns a string which is before a string. For example: I want to get the string along with spaces which resides before "15/08/2017"

a='''S
LINC             SHORT LEGAL                                   TITLE NUMBER
0037 471 661     1720278;16;21                                 172 211 342

LEGAL DESCRIPTION
PLAN 1720278  
BLOCK 16  
LOT 21  
EXCEPTING THEREOUT ALL MINES AND MINERALS  

ESTATE: FEE SIMPLE  
ATS REFERENCE: 4;24;54;2;SW

MUNICIPALITY: CITY OF EDMONTON

REFERENCE NUMBER: 172 023 641 +71

---------------------------------------------------------------------------- 
----
                 REGISTERED OWNER(S)
 REGISTRATION    DATE(DMY)  DOCUMENT TYPE      VALUE           CONSIDERATION
 --------------------------------------------------------------------------- 
 -- 
 ---

 172 211 342    15/08/2017  AFFIDAVIT OF        $610,000      CASH & MTGE'''
                            SURVIVING JOINT  
                            TENANT                         

should return

172 211 342

Here is the expression I have pieced together so far:

re.match("(.*?)15/08/2017",a).group()

It returns

'172 211 342    15/08/2017'