Regex pattern for mm/dd/yyyy and mmddyyyy in Scala-CodePudding

I have date in my .txt file which comes like either of the below:

mmddyyyy

mm/dd/yyyy

Below is the regex which works fine for mm/dd/yyyy.

^02\/(?:[01]\d|2\d)\/(?:19|20)(?:0[048]|[13579][26]|[2468][048])|(?:0[13578]|10|12)\/(?:[0-2]\d|3[01])\/(?:19|20)\d{2}|(?:0[469]|11)\/(?:[0-2]\d|30)\/(?:19|20)\d{2}|02\/(?:[0-1]\d|2[0-8])\/(?:19|20)\d{2}$

However, unable to build the regex for mmddyyyy. I just want to understand is there any generic regex that would work for both cases?

CodePudding user response：

If you want to match all those variations with either 2 forward slashes or only digits, you can use a positive lookahead to assert either only digits or 2 forward slashes surrounded by digits.

Then in the pattern itself you can make matching the / optional.

Note that you don't have to escape the \/

^(?=\d (?:/\d /\d )?$)(?:02/?(?:[01]\d|2\d)/?(?:19|20)(?:0[048]|[13579][26]|[2468][048])|(?:0[13578]|10|12)/?(?:[0-2]\d|3[01])/?(?:19|20)\d{2}|(?:0[469]|11)/?(?:[0-2]\d|30)/?(?:19|20)\d{2}|02/?(?:[0-1]\d|2[0-8])\?(?:19|20)\d{2})$

Regex demo

Another option is to write an alternation | matching the same pattern without the / in it.

CodePudding user response：

Why use regex for this? Seems like a case of "Now you have two problems"

It would be more effective (and easier to understand) to use a DateTimeFormatter (assuming you are on the JVM and not using scala-js)

import java.time.format.DateTimeFormatter
import java.time.LocalDate

val mdyNoSlash = DateTimeFormatter.ofPattern("MMddyyyy")
val mdyWithSlash = DateTimeFormatter.ofPattern("MM/dd/yyyy")

def attemptParse(rawDate: String) = {
  Try { LocalDate.parse(rawDate, mdyNoSlash) }.orElse {
    Try { LocalDate.parse(rawDate, mdyWithSlash) }
  }.get
}

Testing in REPL

scala> attemptParse("12252022")
val res13: java.time.LocalDate = 2022-12-25

scala> attemptParse("12/25/2022")
val res14: java.time.LocalDate = 2022-12-25

scala> attemptParse("25/12/2022")
java.time.format.DateTimeParseException: Text '25/12/2022' could not be parsed: Invalid value for MonthOfYear (valid values 1 - 12): 25