replace only the first match

I have a txt with 2000 lines. i want to open the txt,scan,find EE1 and replace it(only the first EE1,not the second) then create a new txt and close...

this is something a found but it replace all the EE1 it found.I want to replace only the first match..

 close all;  clear;  clc
 % Set user specific paths and values
% ======================================================================
HFSS_EXE_PATH='C:\Program Files (x86)\Ansoft\HFSS12';
SCRIPT_PATH='c:\HFSS\scriptttt.vbs';
NEW_VBS_FILE='c:\HFSS\scrip.vbs';
% ======================================================================
 cc=0;
 for wg_y=6:1:8
   cc=cc+1;
   % Replace first occurrence of waveguide Y dimension
   %vbs_str=file2str(SCRIPT_PATH);
  fid1=fopen(SCRIPT_PATH,'r+');
 vbs_str=fscanf(fid1,'%c');
 old_str=('Array("NAME:XSize", "Value:=", "EE1"');
new_str=sprintf('Array("NAME:XSize", "Value:=", "%1.0fmm")',wg_y);

new_vbs_str=strrep(vbs_str,old_str,new_str);

   % Create updated VB script file
  fid=fopen(NEW_VBS_FILE,'w');
    fprintf(fid,'%s',new_vbs_str);                                                                               
  fclose(fid);
 end

Thank you...

Answer 1

[Jan]

index   = strfind(vbs_str, old_str);
first   = index(1);
vbs_str = [vbs_str(1:first - 1), new_str, vbs_str(first + length(old_str):end)];

[Andrew]

Τhank you that you answered so quick!...Can you please add your answer to my previous code..I am a little comfused...

[Andrew]

ok i found how to do it!!!!!thank you very much!!!!respect!!!

[Andrew]

 Jan Simon can you help me again? Now i change only the first EE1 with a value.If i want to change the first EE1 i found with a value from a matrix (100,1) the second EE1 i found with a different value from a matrix (100,1) and the third EE1 i found with a different value from a matrix (100,1) ,etc (i have 100 EE1),how can i do it?

Thank you....

[prashanth u]

What is new_str at 3rd line@Jan?

[Jan]

new_str is the variable from the code in the question.