May want to out a warning on this tool that if your checking against a long list of strings you may want to use something else. A massive regex can be a huge performance bottleneck. I know from personally killing a production server by using an inefficient regex on large strings of data.
this post was submitted on 17 Sep 2026
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list2regexp will limit each regular expression to matching 10,000 entries by default. Matching against each pattern of 10,000 entries takes around 1 microsecond on a laptop. I've fed it several megabytes of IP addresses and it doesn't choke. That's kind of the point.
Here is an example. Given the following list of strings:
1.2.3.4
127.0.0.1
127.0.0.2
127.0.0.11
128.0.0.1
125.0.0.*
200.0.0.1
2001:0db8:85a3:0000:0000:8a2e:0370:1300
2001:0db8:85a3:0000:0000:8a2e:0370:1330
2001:0db8:85a3:0000:0000:8a2e:0370:1337
あいうえお
あいうえこ
😃😁😆😐
😃😁😆
😃😁🤣
list2regexp will return the following pattern:
^(?:1(?:(?:\.(?:2(?:\.(?:3(?:\.(?:4))))))|(?:2(?:(?:5(?:\.(?:0(?:\.(?:0(?:\.(?:.*)))))))|(?:7(?:\.(?:0(?:\.(?:0(?:\.(?:(?:1(?:|(?:1)))|(?:2))))))))|(?:8(?:\.(?:0(?:\.(?:0(?:\.(?:1)))))))))))|(?:2(?:0(?:0(?:(?:\.(?:0(?:\.(?:0(?:\.(?:1))))))|(?:1(?::(?:0(?:d(?:b(?:8(?::(?:8(?:5(?:a(?:3(?::(?:0(?:0(?:0(?:0(?::(?:0(?:0(?:0(?:0(?::(?:8(?:a(?:2(?:e(?::(?:0(?:3(?:7(?:0(?::(?:1(?:3(?:(?:0(?:0))|(?:3(?:(?:0|7))))))))))))))))))))))))))))))))))))))))))|(?:あ(?:い(?:う(?:え(?:(?:お|こ))))))|(?:😃(?:😁(?:(?:😆(?:|(?:😐)))|(?:🤣))))$This regexp is literally the worst fucking thing I have ever seen. Please never ever use a regexp of that size ever in your production system.
Congrats Frau Kuh, you got a special FAQ entry in the README just for you
As funny as that would be, this has been a surprisingly common misconception.
This drive-by criticism is the worst fucking thing I have ever seen. Please never ever use drive-by criticism without any code or references which back up its claims to inform your opinion on others work.
The reason this is horrible is because its absolutely horrible if you ever have to manually touch this regex ever again.
This was not meant as a way to criticise your tool, because it is cool what you did there, but using regexp of that size is something I would advise against in an production environment.
So, your criticism is that machine-generated code can be hard to edit when a human needs to touch it. I don't see how that is relevant here, being human-legible or editable is not a stated goal of the project. Rather than lobbing criticism against any software which machine-generates code for machine consumption, it makes more sense to use your best judgement and choose the software which is right for your use-case. I don't even want to know why you would ever machine-generate a regexp from a list, not save the list, and then somehow need to edit the regexp without access to the original items. That's not a software problem.
Yeah its Kind of like that and I do have to admit, that my original comment was bad and that I should have specified it in the original comment.