How to create a regex pattern with specific conditions for spaces and underscores?

I need help creating a regex pattern with the following conditions:

1. It can be the empty string.

2. It can allow alphanumeric characters.

3. For special characters, only space or underscore is allowed.

4. It cannot contain consecutive underscores (_) or spaces ( ).

5. It cannot contain 3 or more underscores (_) or spaces ( ).

6. If underscore (_) or space ( ) is present, the first occurrence of _ or space must be after at least 3 characters. (e.g.: 123_)

7. The second occurrence of _ or space must be before at least 4 alphanumeric characters.

(123_AB_YYYY)--> Valid ---> Since it has 4 alphanumeric after second underscore

(123_AB_YYY)--> InValid Since it has 3 alphanumeric after second underscore

Valid example:

• 123_ABC_ERRR

Not Valid:

• 123__
• 123_VBV_HH_

Can someone help me frame this regex pattern in JavaScript?

Here's a regex that might satisfy your description:

/^(?:[a-z\d]{3,}[_ ]?){0,2}[a-z\d]*$/i

^                    # beginning of the text
  (?:                # non-capturing group 
     [a-z\d]{3,}     # 3 or more alphanumeric (the i flag ensures A-Z can also be matched)
     [_ ]?           # followed by an optional underscore or whitespace
  ){0,2}             # the group can repeat 0 to 2 times, meaning there can only be up to 2 underscore/whitespace exist in your text, and they're separated by at lease 3 alphanumerics
  [a-z\d]*           # any number of alphanumetics after that
$                    # end of the text

See the test cases.

Edit

According to the clarified information, the regex is slightly more complicated:

^(?:[a-z\d]{3,}[_ ](?:[a-z\d]{2,}[_ ][a-z\d]{4,})?)?[a-z\d]*$

^                                  # beginning of the text
  (?:                              # the first optional non-capturing group 
     [a-z\d]{3,}[_ ]               # 3 or more alphanumeric (the i flag ensures A-Z can also be matched), followed by an underscore or whitespace
     (?:                           # the second optional group
       [a-z\d]{2,}[_ ][a-z\d]{4,}  # if the second underscore occurs, it must be preceded by at least 2 alphanumerics, and after it must has at least 4 alphanumerics
     )?                            # the second optional group ends
  )?                               # the first optional group ends
  [a-z\d]*                         # any number of alphanumetics after that
$                                  # end of the text

See the updated use cases