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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

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3
  • 1
    If you use PCRE, try regex101.com/r/LTtcJv/2 Commented Feb 25, 2020 at 1:09
  • Magic! Thank you Wiktor. Commented Feb 25, 2020 at 1:57
  • 1
    Sorry, it is actually much easier, I posted the comment late at night :) Commented Feb 25, 2020 at 7:49

2 Answers 2

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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.
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1 Comment

Thanks Wiktor. Now my requirements have changed a little.. I posted another question separately seeking help. It's a small modification of the challenge here that I think will require the (?(A)X|Y) approach. I would be grateful if you could help?
-1

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:

  1. There are two groups - one for X and one for Y.
  2. 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.
  3. 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.)

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1 Comment

Thank you for your help. But my problem has now changed and I need to use PCRE. Please see this question if you have time?

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