Regex: backtracking and optional quantifier-CodePudding

I have some file names and I want to get the timestamp and optional sequence number out of it.

Log-20220408_212412_001.txt           
Log-20220408_212412.txt    
Log-20220408_2124.txt

I used

Log-(?<timestamp>[0-9_] )(?<sequence>_[\d]{3})?.txt$

In C#, it works if I configure regex to search from "right to left". But the "right to left" flag is not commonly supported. Is there any alternative way to do it? Thanks

CodePudding user response：

You may use this regex with an optional group:

^Log-(?<timestamp>\d (?:_\d{4,})?)(?:_(?<sequence>\d{3}))?\.txt

C# RegEx Demo

RegEx Details:

^: Start
Log-: Match Log-
(?<timestamp>\d _(?:_\d{4,})?): Match and capture digits_digits in names group timestamp where part after _ is optional.
(?:_(?<sequence>\d{3}))?: Optional group that starts with _ and matched and captures 3 digits in named group sequence
\.txt: Match .txt