.NET的条件正则表达式，请检查（条件）{match pattern}是否为其他{match else pattern}

Question

I have to search due date in below sms that also contain billed date.

"Bill dated 27-May-18 for your airtel fixedline/broadband ID ##### has been sent at abc@abc.com from ebill@abc.com. Due amount: Rs 2,358.82, due date: 15-Jun-18"

there could be many different sms in which they may not have billed date or due date mentioned explicitly in the sms. So previously, we were just looking for a date format by regex,

"((\d{4}|\d{2})-(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)-(\d{4}|\d{2}))|((\d{4}|\d{2})-\d{2}-(\d{4}|\d{2}))|((\d{4}|\d{2})\/(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)\/(\d{4}|\d{2}))|((\d{4}|\d{2})\/(\d{3}|\d{2})\/(\d{4}|\d{2}))|([0-3][0-9]((st)|(nd)|(rd)|(th))((\s)?)(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec))" 

It works well when there is only due date and no bill date. But in above sms it always returns bill date instead of due date.

I want to find a solution that it will always return me date like what above regex do or if there is any due date mentioned then it should return me date next to due date.

possible solution I see is

1. check for due date - (due date: )
2. if found return me date next to due date - (?<=due date: ).*
3. else 2 not found then return me any date format that exist using the above mentioned regex

Answer 1

All you have to do is make due date optional at the beginning [\\S\\s]due date .
This will start at the end of the string then backtrack until it finds it
then match the date right after it.

If it's not found, the first date it finds in the string will be matched.

To tell if it matched due date check if group 1 matched.
If it's null, it wasn't matched.

The regex:

@"(?:[\\S\\s]*(due\\s+date\\s*:)\\s*)?((\\d{4}|\\d{2})-(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)-(\\d{4}|\\d{2}))|((\\d{4}|\\d{2})-\\d{2}-(\\d{4}|\\d{2}))|((\\d{4}|\\d{2})/(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)/(\\d{4}|\\d{2}))|((\\d{4}|\\d{2})/(\\d{3}|\\d{2})/(\\d{4}|\\d{2}))|([0-3][0-9]((st)|(nd)|(rd)|(th))((\\s)?)(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec))"

Readable version:

    (?:
         [\S\s]*  
         ( due \s+ date \s* : )        # (1)
         \s* 
    )?

    (                             # (2 start)
         ( \d{4} | \d{2} )             # (3)
         -
         (                             # (4 start)
              Jan
           |  Feb
           |  Mar
           |  Apr
           |  May
           |  Jun
           |  Jul
           |  Aug
           |  Sep
           |  Oct
           |  Nov
           |  Dec
         )                             # (4 end)
         -
         ( \d{4} | \d{2} )             # (5)
    )                             # (2 end)
 |  (                             # (6 start)
         ( \d{4} | \d{2} )             # (7)
         - \d{2} -
         ( \d{4} | \d{2} )             # (8)
    )                             # (6 end)
 |  (                             # (9 start)
         ( \d{4} | \d{2} )             # (10)
         /
         (                             # (11 start)
              Jan
           |  Feb
           |  Mar
           |  Apr
           |  May
           |  Jun
           |  Jul
           |  Aug
           |  Sep
           |  Oct
           |  Nov
           |  Dec
         )                             # (11 end)
         /
         ( \d{4} | \d{2} )             # (12)
    )                             # (9 end)
 |  (                             # (13 start)
         ( \d{4} | \d{2} )             # (14)
         /
         ( \d{3} | \d{2} )             # (15)
         /
         ( \d{4} | \d{2} )             # (16)
    )                             # (13 end)
 |  (                             # (17 start)
         [0-3] [0-9] 
         (                             # (18 start)
              ( st )                        # (19)
           |  ( nd )                        # (20)
           |  ( rd )                        # (21)
           |  ( th )                        # (22)
         )                             # (18 end)
         (                             # (23 start)
              ( \s )?                       # (24)
         )                             # (23 end)
         (                             # (25 start)
              Jan
           |  Feb
           |  Mar
           |  Apr
           |  May
           |  Jun
           |  Jul
           |  Aug
           |  Sep
           |  Oct
           |  Nov
           |  Dec
         )                             # (25 end)
    )                             # (17 end)

Output

 **  Grp 0 -  ( pos 0 : len 159 ) 
Bill dated 27-May-18 for your airtel fixedline/broadband ID ##### has been sent at abc@abc.com from ebill@abc.com. Due amount: Rs 2,358.82, due date: 15-Jun-18  
 **  Grp 1 -  ( pos 140 : len 9 ) 
due date:  
 **  Grp 2 -  ( pos 150 : len 9 ) 
15-Jun-18  
 **  Grp 3 -  ( pos 150 : len 2 ) 
15  
 **  Grp 4 -  ( pos 153 : len 3 ) 
Jun  
 **  Grp 5 -  ( pos 157 : len 2 ) 
18  
 **  Grp 6 -  NULL 
 **  Grp 7 -  NULL 
 **  Grp 8 -  NULL 
 **  Grp 9 -  NULL 
 **  Grp 10 -  NULL 
 **  Grp 11 -  NULL 
 **  Grp 12 -  NULL 
 **  Grp 13 -  NULL 
 **  Grp 14 -  NULL 
 **  Grp 15 -  NULL 
 **  Grp 16 -  NULL 
 **  Grp 17 -  NULL 
 **  Grp 18 -  NULL 
 **  Grp 19 -  NULL 
 **  Grp 20 -  NULL 
 **  Grp 21 -  NULL 
 **  Grp 22 -  NULL 
 **  Grp 23 -  NULL 
 **  Grp 24 -  NULL 
 **  Grp 25 -  NULL