DART Conditional find and replace using Regex

Question

I have a string that sometimes contains a certain substring at the end and sometimes does not. When the string is present I want to update its value. When it is absent I want to add it at the end of the existing string.

For example:

int _newCount = 7;
_myString = 'The count is: COUNT=1;'
_myString2 = 'The count is: '
_rRuleString.replaceAllMapped(RegExp('COUNT=(.*?)\;'), (match) {

//if there is a match (like in _myString) update the count to value of _newCount
//if there is no match (like in _myString2) add COUNT=1; to the string

}

I have tried using a return of:

return "${match.group(1).isEmpty ? _myString + ;COUNT=1;' : 'COUNT=$_newCount;'}";

But it is not working.

Since string #2 has no match, no replacement can be done. Do you want to match COUNT=<digits> after count is:? Is count is: string always present? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jul 29, 2020 at 8:25
count is: is not a reliable match as it may change. If no match is found for 'COUNT=<digits>; I want to add it to the string. — dwax
– dwax, Commented Jul 29, 2020 at 8:30
One more: do you expect COUNT=<digits>; always at the end of string? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jul 29, 2020 at 8:41

Wiktor Stribiżew · Accepted Answer · 2020-07-29 09:01:34Z

Note that replaceAllMatched will only perform a replacement if there is a match, else, there will be no replacement (insertion is still a replacement of an empty string with some string).

Your expected matches are always at the end of the string, and you may leverage this in your current code. You need a regex that optionally matches COUNT= and then some text up to the first ; including the char and then checks if the current position is the end of string.

Then, just follow the logic: if Group 1 is matched, set the new count value, else, add the COUNT=1; string:

The regex is

(COUNT=[^;]*;)?$

See the regex demo.

Details

(COUNT=[^;]*;)? - an optional group 1: COUNT=, any 0 or more chars other than ; and then a ;
$ - end of string.

Dart code:

_myString.replaceFirstMapped(RegExp(r'(COUNT=[^;]*;)?$'), (match) {
     return match.group(0).isEmpty ? "COUNT=1;" : "COUNT=$_newCount;" ; }
)

Note the use of replaceFirstMatched, you need to replace only the first match.

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