TL;DR: if you’re having trouble matching whitespace, look out for non-breaking spaces -
/ U+00A0
. If you’re using regex101.com to debug your pattern, be aware that if you paste non-breaking spaces into the regex101.com Test String field, it will silently turn them into regular spaces. Good times. 
I’ve just spent an over hour trying to match whitespace, and only now realized that there are non-breaking spaces in my input.
Goal
I want to read some variables from a text input for use in later Workflow steps. The input is originally from an email sent by a SAP module, which I’ve read using the Microsoft 365 plugin, and then pass through the HTML module to strip out tags/scripts, leaving me with just plain text.
The portion of input I’m testing looks like this:
Connection ID: C-999999 Leave Date: 05/13/2022
What I tried
I’m normally happy writing my regex without using additional tools. However, if I get stuck, I’ll lean on a utility like regex101.com because it can point out mistakes in my pattern I might be missing, and the step-by-step debug feature is really useful to see exactly where your pattern [doesn’t] match. If you’re reading this and you haven’t seen it, it’s well worth two minutes of your time to check it out.
Anyway, when I copied the string from the output of the HTML plugin step and pasted into regex101.com, the following pattern matched fine:
/C-(\d{5,}/
and that worked OK in the Pattern Match step as:
{{connection_id:/C-{{employee_connection_id:/\d{5,}/}}
When I added a single [normal] space onto the end of the pattern on regex101.com, it matched fine:
/C-(\d{5,} /
If my pattern matched the data in the regex101.com Test String field, the pattern must be correct for my data and should work inside the Insight Connect Pattern Match step, right? Wrong! 
Problem
When I brought the pattern back into the Pattern Match step, it failed every time:
{{connection_id:/C-{{employee_connection_id:/\d{5,} /}}
Underlying cause
After a while of trying various things which didn’t work, I decided to take a look at the input to the Pattern Match step using browser developer tools. Of course, it looks like this:
Connection ID: C-999999 Leave Date: 05/13/2022
The original email was full of non-breaking spaces, which don’t match either / / or /\s/ but look identical to a regular space on screen.
Solution
I tried adding a non-breaking space to the end of my pattern, and now it matches just fine:
C-{{connection_id:/\d{5,}/}}\x{A0}
I only showed two fields here, but my regex will be matching 15+ fields when it’s fully built. To keep things simpler, I’ve added a preceding Advanced Regex stage to replace all non-breaking space with a standard space. Since the Advanced Regex plugin uses Python syntax rather than RE2, the match pattern for a non-breaking space looks like this:
\u00A0
Wish I’d thought of all this an hour ago, but at least I know now. Hope this helps someone at some point.