Search for word after specified character

I have a text file that contains several words
...........
...........  dimension="13.7mm" row="30" speed="13.4" ........


I want to extract the value 13.7mm
This is what I have done 
//loaded the file
string file = "c:\\temp\\myTextFile.txt";
string searchText = "dimension=";
//Declare reader as a new StreamReader with file as the file to use  

    StreamReader reader = StreamReader(file);  

    
//Declare text as the reader reading to the end  

     String text = reader.ReadToEnd();  

     //If the searchText is a match  

     if (Regex.IsMatch(text, searchText))  

     {
        
         MessageBox.Show("found it");

       //OK now that I found the word 'dimension="
       //how to display the next characters between the next "  and " 
       //the number of characters may vary (e.g. 125.7mm)
            
    }

sring[] lines = Regex.Split("server=", " ");

         foreach (string line in lines)
         {
             listBox1.Items.Add(line);
            
         }