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Adding an additional letter matching group to an existing regex

I have the following regex: (?:\/us)?\/[a-z]{2}[_-][a-z]{2}(?:\/?$|(?=\/))|\/[a-z]{2}(?:\/?$|(?=\/))^([a-z]{2}\/retail)
As you can see, it's not particularly easy on the eyes. You can see it in action here: https://regex101.com/r/4AZwuP/1 (enable substitutions to see the desired result - the removal of matches)
Here's a few entries it's supposed to match:
/us/en_us/retail/en (matches /us/ and /us/en_us/)
/us/en_us/retail (matches /us/ and /en_us/)
/gb/en_gb/retail/en-uk (matches /en_gb and /en-uk)
Note that, these are just prefixes and the full url might look something like:
/de/de_de/retail/de_de/products/catalog
The goal is to run the regex and delete matches so that this lines becomes:
de/retail/products/catalog
The above Regex accomplishes this with one exception: in the first example, I need it to match not only /us/en_us but also /en (or /de or /mx - in other words, there's an additional country code there; it unfortunately does not.
What I do know for a fact is that if those two characters are present, it'll be one of these two:
.../retail/en
.../retail/en/something/or/other
In either case it's always two characters either alone or followed by a forward slash.
How can I modify the original regex to deal with this annoying edge case?
Bonus: how does the original work?

If a lookbehind is supported you might use:
(?:\/[a-z]{2})?\/[a-z]{2}[-_][a-z]{2}\b|(?<=\/retail)\/[a-z]{2}\b
(?:\/[a-z]{2})? Optionally match / and 2 chars a-z
\/[a-z]{2}[-_][a-z]{2}\b Match / 2 chars a-z. Then either - or _ and 2 char a-z
| Or
(?<=\/retail)\/[a-z]{2}\b Match 2 chars a-z asserting /retail directly to the left
Regex demo
Or use a capture group, and in the callback of replace check if group 1 exists. If it does, use it in the replacement to keep it.
(?:\/[a-z]{2})?\/[a-z]{2}[-_][a-z]{2}\b|\/(retail)\/[a-z]{2}\b
Regex demo

I suppose you want remove country code.then the begin /gb is country code also.
My regex is this (\/\w{2}(?=\/|$))|(\/\w{2}(-|_)\w{2}(?=\/|$))
let break in into two regex
(\/\w{2}(?=\/|$)) match two letter after / and end with / or nothing
(\/\w{2}(-|_)\w{2}(?=\/|$)) match two letter plus _|- and plus two letter,also start with / end with /
it match all example in your regex101,but it will failed if there has other two letters in your url

Regex to match certain pattern unless part of larger pattern

I want to write a regex to find whenever there isn't a space after a period or comma, unless there are three periods in a row, in which case it's find to have another word character after.
E.g.
"Hello.how are you" -> Match
"Sounds good..." -> No Match
"...are you sure?" -> No Match
This is what I have so far (quotation marks after ., are fine too), but it finds a match for the third example, which I don't want.
/[.,][^\s"”.]/g
How can I specify that \.\.\.\w should not match, but any other \.\w should?
(Using js)

I think the following regex does what you need:
(?<!\.\.)[.,](?=[^\s.])
It matches any period that isn't preceeded by two periods and that isn't followed by a period or a whitespace.

javascript regular expression allow name with one space and special Alphabets

how to write regular expression allow name with one space and special Alphabets?
I tried with this [a-zA-Z]+(?:(?:\. |[' ])[a-zA-Z]+)* but not working for me,
example string Björk Guðmundsdóttir

You may try something along these lines:
^(?!.*[ ].*[ ])[ A-Za-zÀ-ÖØ-öø-ÿ]+$
The first negative lookahead asserts that we do not find two spaces in the name. This implies that at most one space is present (or no spaces at all). Then, we match any number of alphabets, with most accented letters included. Spaces can also be matched, but the lookahead would already ensure that at most one space can be present.
Demo

Use this one:
[a-zA-Z\u00C0-\u00ff]*[ ]{1}[a-zA-Z\u00C0-\u00ff]*
Answer from other question

RegEx matching help: won't match on each appearence

I need to write a little RegEx matcher which will match any occurrence of strings in the form of
[a-zA-Z]+(_[a-zA-Z0-9]+)?
If I use the regex above it does match the sections needed but would also match onto the abc part of 4_abc which is not intended. I tried to exclude it with:
(?:[^a-zA-Z0-9_]|^)([a-zA-Z]+(_[a-zA-Z0-9]+)?)(?:[^a-zA-Z0-9_]|$)
The problem is that the 'not' matches at the beginning and end are not really working like I hoped they would. If I use them on the example
a_d Dd_da 4_d d_4
they would block matching the second Dd_da because the space was used in the first match.Sadly I can't use lookarounds because I am using JS.
So the input:
a_d Dd_da 4_d d_4
should match: a_d, Dd_da and d_4
but matches: a_d (there is a space at the end)
Is there another way to match the needed sections, or to not consume the 'anchor' matches?
I really appreciate your help.

You can make use of \b:
\b[a-zA-Z]+(_[a-zA-Z0-9]+)?\b
\b matches the (zero-width) point where either the preceding character or following character is a letter, digit or underscore, but not both. It also matches with the start/end of the string if the first/last character is a letter, digit or underscore.

Regex - must contain number and must not contain special character

I want to check by regex if:
String contains number
String does not contain special characters (!<>?=+#{}_$%)
Now it looks like:
^[^!<>?=+#{}_$%]+$
How should I edit this regex to check if there is number anywhere in the string (it must contain it)?

you can add [0-9]+ or \d+ into your regex, like this:
^[^!<>?=+#{}_$%]*[0-9]+[^!<>?=+#{}_$%]*$
or
^[^!<>?=+#{}_$%]*\d+[^!<>?=+#{}_$%]*$
different between [0-9] and \d see here

Just look ahead for the digit:
var re = /^(?=.*\d)[^!<>?=+#{}_$%]+$/;
console.log(re.test('bob'));
console.log(re.test('bob1'));
console.log(re.test('bob#'))
The (?=.*\d) part is the lookahead for a single digit somewhere in the input.

You only needed to add the number check, is that right? You can do it like so:
/^(?=.*\d)[^!<>?=+#{}_$%]+$/
We do a lookahead (like peeking at the following characters without moving where we are in the string) to check to see if there is at least one number anywhere in the string. Then we do our normal check to see if none of the characters are those symbols, moving through the string as we go.
Just as a note: If you want to match newlines (a.k.a. line breaks), then you can change the dot . into [\W\w]. This matches any character whatsoever. You can do this in a number of ways, but they're all pretty much as clunky as each other, so it's up to you.