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Observe this question (which will be shortly deleted but is being kept for observation).

It has a tag of [#]. The sidebar says [#] x 2, but clicking on it doesn't work too well.

So now I'm left to wonder if there is, in fact, another # question. But should this even be allowed in the first place? I can't think of any legitimate tags that don't include at least one letter or number (404 being an example of a legitimate tag with no letters).

Tagging this question as #, ., &, and -.

Edit: Those tags were stripped out. Which now turns this into a question of, how did that get there?

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Sorry for all of the edits... –  jjnguy Jul 23 '10 at 1:53
    
the "clicking on it doesn't work too well" part is a dupe, I just haven't found the original yet –  balpha Jul 23 '10 at 5:28
1  
@balpha I think I deleted the original dupe of this because I thought it was crazy.. looks like I was the crazy one –  Jeff Atwood Jul 29 '10 at 5:25
    

3 Answers 3

up vote 3 down vote accepted

This is my bad -- bug in the sanitization routine. There is a new unrolled loop one now.

The rules for tag strings are somewhat complex:

  • max of 25 chars
  • valid tag chars are a-z 0-9 # + - .
  • we automatically convert _ to - wherever we see it
  • tags can't start with - or #
  • tags can't end with - or .
  • + is only valid at the end of a tag
  • wildcard, if allowed, is * (this is for client side use only; server wildcard is ~)
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1  
+ behaves strangely: meta.stackexchange.com/q/102957/155238 –  Willie Wong Aug 18 '11 at 15:43

I can't think of any legitimate tags that don't include at least one letter or number (404 being an example of a legitimate tag with no letters).

I disagree.

It would be awesome to be able to tag things like operators consisting only of symbols ?: >>>= >@> and so on. Especially as Google and most other search engines don't seem to be able to index these tokens. A lot of questions about these operators get repeated over and over, often with a line "I've tried Googling but couldn't find anything".

This feature would actually make me want to use SO's search.

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2  
Good point .... –  Daniel Vassallo Jul 22 '10 at 23:23
1  
You make a good point but by the way the system is currently designed, this is a bug –  Earlz Jul 22 '10 at 23:24
1  
#! might be even more popular than those. –  Larry Wang Jul 23 '10 at 1:24
    
ternary operator! –  Shogging through the snow Jul 23 '10 at 1:54
3  
@Shog9, you should know better! ___ Conditional operator. ___ –  Pops Jul 23 '10 at 2:02
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@Popular: if we knew what it was called, we wouldn't need symbols in tags! –  Shogging through the snow Jul 23 '10 at 2:28
    
@Shog9, right, but I'm sure you do know. And I apologize for the formatting of the last comment, I couldn't get Markdown to do what I wanted. Took five or so edits just to get it to where it is. –  Pops Jul 23 '10 at 2:30
    
What's wrong with the last comment? @Pop Back on-topic (sorta), there was a bit of discussion about this a while back... –  Shogging through the snow Jul 23 '10 at 2:38
    
@Shog9, er, it was supposed to just be a quick amusing comment. I know that newbies do often helplessly ask about it; I would have probably done so myself, if Stack Overflow had existed when I was learning Java. I was working off of the point that you are not such a newbie, but you yelled out "ternary operator" and not "conditional operator" anyways. –  Pops Jul 23 '10 at 2:56
    
@Popular: no worries, I laughed. –  Shogging through the snow Jul 23 '10 at 3:32

I was able to add the [#] tag back in by encasing it in double quotes - "#". If you edit this question the quotes are not present in the tags field, however the hash / pound sign is still there.

I also tried to add the other special characters you mention. With all 4 only the dot (.) tag remained. This is what I entered for tags:

bug "#" "." "&" "-"
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thanks for the repro, this helped –  Jeff Atwood Jul 29 '10 at 5:25

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