How @mention works, how can I find mention during comment in .Net

Question

I need to know how mention works, how should find mentions during a text. are we must to find first of '@' and last of not @"^[a-zA-Z0-9_,]+$"

thank you for sharing your experience

string comment=" hi @fri.tara3^";
mention is : "@fri.tara3"

You already know you can use regex for this. Is there a question somewhere? Where did you get stuck? — Imre Pühvel, May 14 '16 at 07:39
It's really hard to find last index of **not @"^[a-zA-Z0-9_,]+$"** , I don't know what should I do — ara, May 14 '16 at 07:48
@ara: what about [`@"@\S+\b"`](https://regex101.com/r/uH3wA3/1)? — Wiktor Stribiżew, May 14 '16 at 07:59
@WiktorStribiżew this is magical! unbelievable , thank you man — ara, May 14 '16 at 08:09
@WiktorStribiżew - while the principle is valid, `\S` is not the same as `[a-zA-Z0-9_,]` and neither is `\b` matching the negation class. — Imre Pühvel, May 14 '16 at 08:23

Imre Pühvel · Accepted Answer · 2016-05-14T08:30:07.250

Looks like a good fit for regular expressions. There are multiple ways to solve this.

Here's the simplest one:

 (?<mention>@[a-zA-Z0-9_.]+)[^a-zA-Z0-9_.]

it searches matching characters followed by non-matching character. [^ ... ] does the negation bit
(?<mention> ... ) declares an explictit group to capture mention without including the non-matching character immediately following the mention.
not that this pattern requires a non-matching character after mention, so if it matters work around that.

A cleaner pattern would use a feature called look-ahead:

@[a-zA-Z0-9_.]+?(?![a-zA-Z0-9_.])

(?!) is negative lookahead. Meaning "only match if it is NOT followed by this"
named capture not required as lookahead does not consume the lookahead part.
It supports multiple mention lookups by adding using non-greedy quantifier +?. This ensures that matched mention is as short as possible.

Lookaheads are a tad less known and may become a pain to read if pattern grows too long. But it is a useful tool to know.

Full example using C#:

string comment = "hi @fri.tara3^ @hjh not a mention @someone";
const String pattern = "@[a-zA-Z0-9_.]+?(?![a-zA-Z0-9_.])";
var matches = Regex.Matches(comment, pattern);

for (int i = 0; i < matches.Count; i++)
{
    Console.WriteLine(matches[i].Value);
}

thank you , how about finding multi @ like `"hi @fri.tara3^ @hjh"` am I must to use something like this: `comment=comment.Substring(indexofLastFind,...)` — ara, May 14 '16 at 08:24
Regex can find all matches for you in one pass. See updated example in answer. — Imre Pühvel, May 14 '16 at 08:31
you are magical, unbelievable ! thank you, thank you very much :* — ara, May 14 '16 at 08:37

How @mention works, how can I find mention during comment in .Net

1 Answers1