I'm currently writing a paper for my study on the applicability of Spring Batch (a java framework) for the use cases of the department I'm currently in.

For this, I'm citing parts of the Spring documentation, and I wanted to use a . in the bibliography entry identifier. While this at first had me worried that dots might not be permitted in identifiers, it turns out they are.

However, I can't help but wonder which characters are permitted - is it ASCII characters? A subset of ASCII characters (i.e. alphanumeric, ., - and _)? All unicode characters? Some unicode characters?
Also, even if they are valid, are there characters that could create strange errors depending on which context the bibliography entry is referenced in?

No worries, I don't intend to use emojis in my identifiers, I'm just curious as to which characters are valid. Thanks in advance! :)

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    . is good, anything non-US-ASCII is risky if you use pdfLaTeX. A comma is out for obvious reasons, with Biber most brackets are also out. See also tex.stackexchange.com/q/96454/35864, tex.stackexchange.com/q/408530/35864, tex.stackexchange.com/q/243729/35864, tex.stackexchange.com/q/388500/35864. – moewe Aug 14 '18 at 13:49
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    FWIW I usually stick to lowercase a-z (that way I don't have to remember capitalisation), digits, as well as few special characters that I deem safe, namely ., : and -. I can see why would want to use _, + in certain situations, but I would avoid most other ASCII special chars and definitely the non-ASCII range (even with the Unicode engines - what if I have to go back to pdfLaTeX?). – moewe Aug 14 '18 at 13:58
  • Thanks! Overall, your interpretation overlaps with my use so far - I've been mainly using letters and : so far (due to some advice I read at some point), but being a java developer, I figured that . would be more intuitive. – PixelMaster Aug 14 '18 at 14:02
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    no, you can close it :) – PixelMaster Aug 14 '18 at 14:22
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    @moewe What is valid as a BibLaTeX entry key? – PixelMaster Aug 14 '18 at 14:40