StringCases and DatePattern: something strange

Question

May be that I did not understand something about Mathematica pattern matching, but I noticed what follow:

st is a string, inside the string there is a date (Italian format) so I picked this code to extract the date:

st = "aaa ddd f f 20/06/2018 ppp mm l l ooo";

pattern = {"Day", "/", "Month", "/", "Year"};

dateTime = StringCases[st, ___ ~~ x : DatePattern[pattern] ~~ ___ :> x]

{"20/06/2018"}

Every seems to work, but if the day is 21 or 22 or 23 the command gives a wrong day:

someStringDate = {"01/06/2018", "02/06/2018", "10/06/2018", "11/06/2018", "12/06/2018", 
                  "20/06/2018", "21/06/2018", "22/06/2018", "30/06/2018", "31/06/2018"};

pattern = {"Day", "/", "Month", "/", "Year"};

dateTime = 
 StringCases[
  someStringDate, ___ ~~ x : DatePattern[pattern] ~~ ___ :> x]

{{"1/06/2018"}, {"2/06/2018"}, {"10/06/2018"}, 
{"1/06/2018"}, {"2/06/2018"}, {"20/06/2018"}, {"1/06/2018"}, 
{"2/06/2018"}, {"30/06/2018"}, {"1/06/2018"}}

Have you noticed what happens? Does it happen to you too?

PS: I forgot to add the version info

$Version

"11.3.0 for Microsoft Windows (64-bit) (March 7, 2018)"

Carl Woll · Accepted Answer · 2018-05-28 15:13:57Z

6

Use Shortest to find the match where DatePattern is longest:

StringCases[
    someStringDate,
    Shortest[___] ~~ DatePattern[pattern] ~~ ___
]

{{"01/06/2018"}, {"02/06/2018"}, {"10/06/2018"}, {"11/06/2018"}, {"12/06/2018"}, {"20/06/2018"}, {"21/06/2018"}, {"22/06/2018"}, {"30/06/2018"}, {"31/06/2018"}}

answered May 28, 2018 at 15:13

Carl Woll

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$\begingroup$ Ok, it works. Thanks very much. $\endgroup$

msalese
– msalese

2018-05-28 15:18:58 +00:00
Commented May 28, 2018 at 15:18
$\begingroup$ I'd add that if the date pattern is inside a string , above code not work. try to apply the solution to string = "Option FTSE MIB 1st Friday W 15/06/2018 CALL 21700" , not extract the date. $\endgroup$

msalese
– msalese

2018-05-28 16:20:22 +00:00
Commented May 28, 2018 at 16:20

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Alexey Popkov · Accepted Answer · 2018-05-29 08:30:57Z

Since the format of the date is explicitly known, you could construct your own DatePattern:

dp = DigitCharacter ~~ DigitCharacter;
myDatePattern = dp ~~ "/" ~~ dp ~~ "/" ~~ dp ~~ dp;

someStringDate = {"01/06/2018", "02/06/2018", "10/06/2018", 
   "11/06/2018", "12/06/2018", "20/06/2018", "21/06/2018", 
   "22/06/2018", "30/06/2018", "31/06/2018", 
   "Option FTSE MIB 1st Friday W 15/06/2018 CALL 21700", 
   "aaa ddd f f 20/06/2018 ppp mm l l ooo"};

StringCases[someStringDate, myDatePattern]

{{"01/06/2018"}, {"02/06/2018"}, {"10/06/2018"}, {"11/06/2018"}, 
{"12/06/2018"}, {"20/06/2018"}, {"21/06/2018"}, {"22/06/2018"}, 
{"30/06/2018"}, {"31/06/2018"}, {"15/06/2018"}, {"20/06/2018"}}

For higher performance you could use RegularExpression:

myDatePattern2 = RegularExpression["\\d{2}/\\d{2}/\\d{4}"];
StringCases[someStringDate, myDatePattern2]

{{"01/06/2018"}, {"02/06/2018"}, {"10/06/2018"}, {"11/06/2018"}, 
{"12/06/2018"}, {"20/06/2018"}, {"21/06/2018"}, {"22/06/2018"}, 
{"30/06/2018"}, {"31/06/2018"}, {"15/06/2018"}, {"20/06/2018"}}

Note that without the redundant BlankNullSequences everything works with DatePattern by default:

pattern = {"Day", "/", "Month", "/", "Year"};
StringCases[someStringDate, DatePattern[pattern]]

{{"01/06/2018"}, {"02/06/2018"}, {"10/06/2018"}, {"11/06/2018"}, 
{"12/06/2018"}, {"20/06/2018"}, {"21/06/2018"}, {"22/06/2018"}, 
{"30/06/2018"}, {"31/06/2018"}, {"15/06/2018"}, {"20/06/2018"}}

Thanks very much, I've implemented the Alexey Popkov hint , and I found that the slowdown is not due to regularExp rather than the ExceLink component when writing to excel. — msalese
– msalese, Commented May 29, 2018 at 12:44

msalese · Accepted Answer · 2018-05-28 16:31:33Z

1

To adapt the code to the situation in which string is like that

string = "Option FTSE MIB 1st Friday W 15/06/2018 CALL 21700"

I modified the command (as suggested by Carl Woll) in this way:

dateTime = StringCases[string,Shortest[___] ~~ x : DatePattern[pattern] ~~ ___ :> x]

It works but is very slow when applied on a long string-list.

edited May 28, 2018 at 16:31

answered May 28, 2018 at 16:24

msalese

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