正则表达式匹配字符串，除非后跟＃

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');