Why is #regex used in task.json in Azure DevOps extension? What does it check for?

Question

I came across this and was wondering what this means and how it works? What's the significance of using #regex here and how does it expand?

https://github.com/microsoft/azure-pipelines-tasks/blob/master/Tasks/DownloadPackageV0/task.json

 "endpointUrl": "{{endpoint.url}}/{{ **#regex ([a-fA-F0-9\\-]+/)[a-fA-F0-9\\-]+ feed }}_apis**/Packaging/Feeds/{{ **#regex [a-fA-F0-9\\-]*/([a-fA-F0-9\\-]+) feed** }}{{#if view}}@{{{view}}}{{/if}}/Packages?includeUrls=false"

Also I would like to know how many packages will it return and display in the Task input UI dropdown if there are thousands of packages in the feed. Is there a known limit like first 100 or something?

Answer 1

#regex doesn't appear to actually be documented anywhere, but it takes two space-delimited arguments. The first is a regular expression and the second is a "path expression" identifying what value to match against, in this case the value of the feed input parameter. If the regex matches the value, it returns the first capturing subexpression, otherwise it returns the empty string.

In this particular context, the feed parameter is formatted as 'projectId/feedId', where projectId and feedId are GUIDs, and projectId and the / are eliminated for organization-scoped feeds (i.e. feeds that are not inside a project). The first regex therefore extracts the project ID and inserts it into the URL, and the second regex extracts the feed ID and inserts it into the URL.

As of this writing, the default limit on the API it's calling is 1000.

Answer 2

Regex stands for regular expression, which allows you to match any pattern rather than an exact string. You can find more info on how to use it in Azure Devops here

This regex is very specific. In this case, the regex ([a-fA-F0-9\\-]+/)[a-fA-F0-9\\-]+\ matches one or more of the following 1) letters a-f (small or capital) Or 2) \ Or 3) - followed by / and then again one or more of those characters.

You can copy the regex [a-fA-F0-9\\-]+/)[a-fA-F0-9\\-]+ into https://regexr.com/ to play around with it, to see what does and doesn't match the pattern.
Examples:

• it matches: a/a a/b abcdef-\/dcba
• but doesn't match: /a, abcdef, this-doesn't-match

Note that the full endpoint consists of concatenations of both regular expression and hardcoded strings!