How to make an If Then Else Regex conditional statement - regex

Question

I am trying to make an If-Then-Else conditional statement in regular expressions.
The regex takes as input a string representing a filename.
Here are my test strings...
The Edge Of Seventeen 2016 720p.mp4
20180511 2314 - Film4 - Northern Soul.ts
20150526 2059 - BBC Four - We Need to Talk About Kevin.ts
In the first string, 2016 represents a year but in the other two strings 2314 and 2059 represent times in 24 hour clock format.
The filename should be retained unchanged if it matches this regex:
\d{8} \d{4} -.*?- .*?\.ts
Which I have tested and it works. It can match these test strings:
20180511 2314 - Film4 - Northern Soul.ts
20150526 2059 - BBC Four - We Need to Talk About Kevin.ts
If the filename does not match that first regex then this regex should be applied to it:
(.*[^ _\,\.\(\)\[\]\-])[ _\.\(\)\[\]\-]+(19[0-9][0-9]|20[0-9][0-9])([ _\,\.\(\)\[\]\-]|[^0-9]$)?
This is a cleandatetime regexp that is used by Kodi to remove everything from a string AFTER a four digit number, if it exists, representing a date between 1900 and 2099. I have also tested this and it works.
Here is what I have tried to make the If-Then-Else Regex but it doesn't work:
I use this format --> (?(A)X|Y)
(?(\d{8} \d{4} -.*?- .*?\.ts)^.*$|(.*[^ _\,\.\(\)\[\]\-])[ _\.\(\)\[\]\-]+(19[0-9][0-9]|20[0-9][0-9])([ _\,\.\(\)\[\]\-]|[^0-9]$)?)
This is A
\d{8} \d{4} -.*?- .*?\.ts
This is X
^.*$
This is Y
(.*[^ _\,\.\(\)\[\]\-])[ _\.\(\)\[\]\-]+(19[0-9][0-9]|20[0-9][0-9])([ _\,\.\(\)\[\]\-]|[^0-9]$)?
This is the expected output...
Test string:
The Edge Of Seventeen 2016 720p.mp4
Expected output:
"The Edge Of Seventeen 2016 " (quotes only included to show that a trailing space can be left at the end)
Test String:
20180511 2314 - Film4 - Northern Soul.ts
Expected output:
20180511 2314 - Film4 - Northern Soul.ts
Test String:
20150526 2059 - BBC Four - We Need to Talk About Kevin.ts
Expected output:
20150526 2059 - BBC Four - We Need to Talk About Kevin.ts
I am looking for a solution entirely in regular expression syntax. Can someone help me to make it work please?
Cheers,
Flex

Answer 1

You may use a PCRE pattern like
^(?!\d{8} \d{4} -.*?- .*?\.ts$)(.*[^ _,.()\[\]-][ _.()\[\]-]+(?:19|20)[0-9]{2})(?:[ _,.()\[\]-]|[^0-9]$)?.*
Replace with $1, see the regex demo.
It matches
^ - start of string
(?!\d{8} \d{4} -.*?- .*?\.ts$) - the negative lookahead fails the match if the whole string matches
\d{8} \d{4} - 8 digits, space, 4 digits, space
-.*?- .*? - -, then any 0 or more chars other than line break chars, as few as possible, - and a space and then again 0 or more chars other than line break chars, as few as possible
\.ts$ - .ts at the end of string
(.*[^ _,.()\[\]-][ _.()\[\]-]+(?:19|20)[0-9]{2})(?:[ _,.()\[\]-]|[^0-9]$)?.*: an optional Group 1 and then the rest of the string:
.* - any 0+ chars other than line break chars as many as possible
[^ _,.()\[\]-] - a char other than
[ _.()\[\]-]+ - 1+ spaces, _, ., (, ), [, ] or -
(?:19|20) - 19 or 20
[0-9]{2} - two digits
(?:[ _,.()\[\]-]|[^0-9]$)? - an optional non-capturing group matching a space, _, ., (, ), [, ] or - or any char other than digit at the end of the string.
.*[^ _,.()\[\]-][ _.()\[\]-]+(?:19|20)[0-9]{2})(?:[ _,.()\[\]-]|[^0-9]$
.* - any 0+ chars other than line break chars as many as possible.

Answer 2

Since you have mentioned that A, X and Y are tested and found working, and since there are only 2 patterns, I think this pattern will work (Python style):
pattern = "(.?(?=" + A + ")" + X + ")|(" + Y + ")"
which means:
(.?(?=A)X)|(Y)
Explanation:
There are two groups - one for X and one for Y.
The group for capturing X starts with .? just to make the engine start moving and check if there is a part matching X ahead (a lookahead). If yes, it continues with matching X since it will encounter it after the lookahead block.
If in (2), the lookahead doesn't match, then the | (or) part, which is Y will take over. If that matches, you get a result. Else, no output.
(Sadly, the patterns for A and Y you posted were not working for me on Python, so I replaced them with my own for testing. Please do confirm if the pattern is working with the original ones.)