fix: false positive for Topcoder

[simplyNour]

The PR fixes the premature regex check for usernames on Topcoder to allow multiple occurrences of underscores and alphanumeric characters.
Fixes: #2669

[github-actions]

Automatic validation of changes

Target	F+ Check	F- Check
Topcoder	✔️   Pass	✔️   Pass

[ppfeister]

Were the F+s due to single-letter handles? Since that seems to be the main change, here.

If so, would it be worth specifying a minimum username length? {3,} instead of + for min 3, for instance.

[akh7177]

Were the F+s due to single-letter handles? Since that seems to be the main change, here.

If so, would it be worth specifying a minimum username length? {3,} instead of + for min 3, for instance.

According to what I observed, even without any changes TopCoder was working correctly, without any F+ or F-