Q: htaccess issue including period/fullstop (.) I currently have this .htaccess rule the works fine:
 RewriteRule ^instructor/([A-Za-z0-9-]+)$ instructor.php?username=$1 [NC,L]

However, when I attempt to add a period into the mix a lot of the rules on the site break so I am assuming the character isn't escaped correctly:
RewriteRule ^instructor/([A-Za-z0-9-\.]+)$ instructor.php?username=$1 [NC,L]

Anyone point me in the right direction please?
Update
It appears to be something to do with the directory structure.
Another selection of rules that apply to this site are the following:
## Registration
RewriteRule ^instructor/register/?$ instructor-form/index.php [L]
RewriteRule ^instructor/register/stage([1-5]+)$ instructor-form/stage$1.php [L]

These work fine (the directory here is instructor-form/
However, there is also a directory called instructor/ which these rules point to:
RewriteRule ^instructor/dashboard/?$ instructor/index.php [L]
RewriteRule ^instructor/account-details/?$ instructor/account-details.php [L]
RewriteRule ^instructor/change-password/?$ instructor/change-password.php [L]

These are the rules that are affected when adding the . into the first rule. The rules are all in order and work fine without the . in the [A-Za-z0-9-] char block. When added the physical instructor/ folder seems inaccessible.

A: You wrote so much text in your question but forgot to mention important details: what is actually broken? Because I do not have clear answer for that I will be speculating here based on the information you have provided so far.

RewriteRule ^instructor/([A-Za-z0-9-\.]+)$ instructor.php?username=$1 [NC,L]

The problem with this rule is that it will also rewrite already rewritten php files: instructor/index.php, instructor/account-details.php, instructor/change-password.php etc.
I think you are relaying on [L] flag too much .. or do not really know how mod_rewrite and [L] flag work. And that is why you are having this issue -- your rule with a dot in pattern rewrites already rewritten URLs.
Useful link: RewriteRule Last [L] flag not working?

You need to add some condition (global rule or condition specific to this rule only) to prevent rewriting already rewritten URLs or existing files.
1. Global rule -- place it somewhere on the top before other rules. Keep in mind that this may not work as intended depending on your website structure and rewrite logic (e.g. when you need to actually rewrite requests to already existing files or folders):
# do not do anything for already existing files
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule .+ - [L]

2. Condition specific to that rule only:
a) do not rewrite if requested URI is physical file
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^instructor/([A-Za-z0-9-\.]+)$ instructor.php?username=$1 [NC,L]

OR
b) do not rewrite .php files
RewriteCond %{REQUEST_URI} !.+\.php$
RewriteRule ^instructor/([A-Za-z0-9-\.]+)$ instructor.php?username=$1 [NC,L]

