not 100% sure what you need, but hopefully you can modify this to work for you:
Code:
<!DOCTYPE html>
<html>
<head>
<title>file reps</title>
</head>
<body style="zoom: 1.5"> <center>
<input type="file" onchange='readText(this)' /> <br />
<textarea rows=10 cols=84 id="txtReps">123456, 123000</textarea> <br />
<button onclick="rip()">run script</button>
<button onclick="output.value=output.defaultValue;">reset</button>
<br />
<textarea rows=10 cols=84 id="output">http://www.mysite.com/file/folder/blah/xxxxxx/_something_else here/zzzzzz/image250
http://www.mysite.com/file/folder/blah/xxxxxx/_something_else here/zzzzzz/image500
http://www.mysite.com/file/folder/blah/xxxxxx/_something_else here/zzzzzz/image800
http://www.mysite.com/file/folder/blah/xxxxxx/_something_else here/zzzzzz/image1000 </textarea>
<script type="text/javascript">
var reader = new FileReader();
function rip(e) {
var out=(e && e.target && e.target.result) || output.value ;
var reps=txtReps.value.split(/\s*,\s*/);
output.value= out.split("\n").map(function(a){
return a .split("xxxxxx").join(reps[0])
.split("zzzzzz").join(reps[1]);
}).join("\n");
}
function readText(that){
if(that.files && that.files[0]){
var reader = new FileReader();
reader.onload = rip;
reader.readAsText(that.files[0]);
}//end if html5 filelist support
}
</script>
</center> </body>
</html>
i budgeted 15 mins, spent 14m37s