docs: Fix section that documents storing regex in variables

What does this MR do?

Fixes the section of our documentation that states that regular expressions can be stored in variables, and comparisons can then be performed against the variables.

Linked to this MR are bug issues raised 7 and 19 months ago. Rather than continuing to document (at best) a buggy feature, I propose removing it from the documentation.

history

11.10 introduced a single line into the docs: (gitlab-foss!18902 (merged), gitlab-foss#43601 (closed))

It is possible perform pattern matching against a variable and regular expression.

A customer tried to make this work, and encountered issues. (ticket link for GitLab team members)

I expanded the documentation to try and make it clear that the feature was limited (!29473 (merged)) and also raised #35438 (closed) to cover at least one case where it did not work.

More recent investigation in #284136 (comment 567847184) indicates that while this feature can appear to work, the circumstances are quite specific.

My interpretation if the thread and the feedback from customers is that the code is insufficient to provide a usable feature.

Quoting @mbobin (#284136 (comment 576859994))

When the regular expression is used as string, it's possible to get a partial match. It also works is the value that it's matching is empty.

But when converted to a regular expression it matches only the full values:

Related issues

• #284136 (closed)
• #35438 (closed)

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