Regex Match Kusto - regex

Question

I have below 2 tables, One with complete list of URLs and other table with regex representation of all URLs (nearly 100 values) with corresponding topic. I now want to create a third table which maps each url with the topic based on the regex pattern.
I figured that kusto offers 'matches regex' but it cannot be used at a row level. Ideally I want to create a function and pass URL which output the corresponding Topic
Table1:
| URL |
Table2:
|URL Regex| Topic|
Output:
|URL | Topic|
let me know if the below logic needs any tuning for it to work,
Query:
.create-or-alter function with findTopic(Path:string) {
toscalar(Table2
| extend TopicName=case (Path matches regex URLRegex, Topic,"Not Found")
| project Topic)
}
Table1
| extend Topic=findTopic(Path)

Answer 1

Regular expressions can't be originated from a dynamic source, like another table. In Kusto, regular expressions must be string scalars.
In your case this isn't a problem, since there are about 100 different topics. You can maintain a stored function that does the URI categorization:
.create-or-alter function GetUrlTopic(Url:string)
{
case(
Url matches regex #"https://bing.com.*", "Search",
Url matches regex #"https://stackoverflow.com.*", "Q&A",
"N/A")
}
Example:
let Uris=datatable(Url:string)
[
"https://bing.com/foo/bar",
"https://bing.com/1/2",
"https://microsoft.com",
"https://stackoverflow.com/q/1",
"https://stackoverflow.com/q/2"
];
Uris
| extend Topic=GetUrlTopic(Url)
Result:
Url
Topic
https://bing.com/foo/bar
Search
https://bing.com/1/2
Search
https://microsoft.com
N/A
https://stackoverflow.com/q/1
Q&A
https://stackoverflow.com/q/2
Q&A