regex match string unless followed by # - regex

Question

I'm trying to add a #param to url's I want to add it to all urls that doesn't already have the # to avoid double param. My urls doesn't look like urls they are made up of handlebar parameters.
they can look like following:
{{app.url}}
{{root.app.url}}
{{app.url}}#param
{{root.app.url}}#param
So I came up with a regex that matches the handlebar tag ({{(root.)?app.url}})
only problem is that when I later uses regexp_replace(url, '({{(root\.)?app\.url}})', '\1#param')
my result looks like this:
{{app.url}}#param
{{root.app.url}}#param
{{app.url}}#param#param
{{root.app.url}}#param#param
One solution I can think of is doing it in two steps, and the 2nd step should look for duplicate #param#param and replace that with single #param.
But it had me wondering if there was a way using regex to exclude the handlebar tags that are followed by # and completely cancel that match?
Here are some examples:
https://regex101.com/r/d3Zyvo/6
Note: this is for use in postgressql update queries. The regex is POSIX/PCRE. I must use regex_replace with back reference since there might be content before and after the hanbdlebar tags, I simply cannot just concatenate the param. (see the link).

Answer 1

You may use a negative lookahead (?!#):
({{(root\.)?app\.url}})(?!#)
^^^^^
See the regex demo.
Details
({{(root\.)?app\.url}}) - Group 1 (later referred to with \1 from the replacement pattern):
{{ - {{ substring
(root\.)? - an optional Group 2 matching 1 or 0 occurrences of root.
app\.url}} - a literal app.url}} substring
(?!#) - a negative lookahead that fails the match if, immediately to the right of the current location, there is a # char.
See Table 9-17. Regular Expression Constraints:
(?!re) negative lookahead matches at any point where no substring matching re begins (AREs only)
PostgreSQL demo:
select regexp_replace('{{app.url}}
{{root.app.url}}
{{app.url}}#param
{{root.app.url}}#param',
'({{(root\.)?app\.url}})(?!#)',
'\1#param',
'g');