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javascriptregexnegative-lookbehind

How to match all strings with a specific REGEX pattern that do not start with a defined character without using a negative lookbehind

I currently have a usecase on which I want to match all http:// and https:// strings in a text but only when they do not start with a " or ' using JavaScript. If they start with another character, e.g., a whitespace, I still only want to match the http:// or https:// without the preceding character.

My current regex uses a negative lookbehind but I just realized that this is not supported in Safari:

/(?<!["'])(https?:\/\/)/gm

So what would be an alternative for using a negative lookbehind to match the following strings in a text:

  • http:// -> should match http://
  • https:// -> should match https://
  • xhttps:// -> should match https:// whereby x can be any character except " and '
  • "https:// -> should NOT match at all
Solution

  • No need of lookbebind here, use character class and groups:

    const vars = ['http://', 'https://', 'xhttps://', '"https://']
const re = /(?:[^'"]|^)(https?:\/\/)/
vars.forEach(x => 
   console.log(x, '- >', (x.match(re) || ['',''])[1])
)

    Regex:

    (?:[^'"]|^)(https?:\/\/)

    EXPLANATION

    --------------------------------------------------------------------------------
  (?:                      group, but do not capture:
--------------------------------------------------------------------------------
    [^'"]                    any character except: ''', '"'
--------------------------------------------------------------------------------
   |                        OR
--------------------------------------------------------------------------------
    ^                        the beginning of the string
--------------------------------------------------------------------------------
  )                        end of grouping
--------------------------------------------------------------------------------
  (                        group and capture to \1:
--------------------------------------------------------------------------------
    http                     'http'
--------------------------------------------------------------------------------
    s?                       's' (optional (matching the most amount
                             possible))
--------------------------------------------------------------------------------
    :                        ':'
--------------------------------------------------------------------------------
    \/                       '/'
--------------------------------------------------------------------------------
    \/                       '/'
--------------------------------------------------------------------------------
  )                        end of \1