Flutter split a string keeping the delimeter

You might split using lookarounds with an alternation | to match both ways before and after the {{...}} part:

\s*(?={{.*?}})|(?<={{.*?}})\s*

The pattern matches:

• \s*(?={{.*?}}) Match optional whitespace chars and assert directly to the right a pattern {{...}}}
• | Or
• (?<={{.*?}})\s* Assert a pattern {{...}}} to the left and match optional whitespace chars

Regex demo | Dart demo

void main() {
    var parts = "this is {{A}} sample {{text}} ...".split(RegExp("\\s*(?={{.*?}})|(?<={{.*?}})\\s*"));
    print(parts);
}

Output

[this is, {{A}}, sample, {{text}}, ...]

An idea to get the result you are looking for, would be:

1. Split the string as you did,
2. find all matches,
3. insert the matches in your splitted string.

See the following sample that illustrates this idea:

void main() 
{
  String a = "this is {{A}} sample {{text}} ...";
  
  var allMatches = RegExp("{{(.*?)}}").allMatches(a);
  
  var parts = a.split(RegExp("{{(.*?)}}"));
  
  for(int i = 0; i < allMatches.length; i++) {

    int newIndex = 2*i+1;
    
    String? newDelimiter = allMatches.elementAt(i).group(0);
    
    
    if (newDelimiter!=null) {
      parts.insert(newIndex,newDelimiter);
    }
    
  }
    
  print(parts); // ["this is ", "{{A}}", " sample ", "{{text}}", " ..."]  
  
}

This worked for me.

(?={{)|(?<=}})

Demo