Regex.Replace：基于可选组的条件替换

Question

I am using Regex.Replace in C#. In the replaced string, I want to put a condition that part of the string only comes out if one of the optional group is captured. For example, I have this regex:-

(?<Expiry>\d+[my])\s+(?<Flag>[a-z][A-Z])?\s*$)

Note that Flag captured group is optional .

The replacement string is

"Expiry is ${Expiry}. Flag is : ${Flag}"

Now I want "Flag is : ${Flag}" string only appear in the result if the Flag group is captured.

I am using following code:

var regex = new
Regex("(?<Expiry>\d+[my])\s+(?<Flag>[a-z][A-Z])?\s*$)",
RegexOptions.IgnoreCase | RegexOptions.Compiled);
string result = regex.Replace("10y", "Expiry is ${Expiry}. Flag is : ${Flag}";

Result should by Expiry is 10y .

string result = regex.Replace("10y abc", "Expiry is ${Expiry}. Flag is ${Flag}");

Result should be "Expiry is 10y. Flag is : abc".

Any help is much appreciated.

Answer 1

You may use different replacement values based on whether the specified group participated in the match (=matched) or not.

See the example below:

var s = "1m aZ";
var pat = @"(?<Expiry>\d+[my])\s+(?<Flag>[a-z][A-Z])?\s*$";
var res = Regex.Replace(s, pat, m => m.Groups["Flag"].Success ? 
    string.Format("Flag is : {0}.", m.Groups["Flag"].Value) : 
    string.Format("Expiry is {0}.", m.Groups["Expiry"].Value));
Console.WriteLine(res);

See the online demo

If Flag group matches, the replacement will be string.Format("Flag is : {0}.", m.Groups["Flag"].Value) , else, the replacement will only contain the expiry message.