Regex back references in Regex.Replace()

Question

I'm using a regex to replace values within some html code. It correctly matches all instances within the html code but when using Regex.Replace() with back references it doesn't replace the back references.

For example

html = "<td>[element]elementreference='oldvalue';[/element]</td>";

html = Regex.Replace(html, @"(['""#(=])" + elementReference.Key + @"(['""#)];|&)", "$1" +   elementReference.Value + "$2", RegexOptions.IgnoreCase);

results in:

"<td>[element]elementreference=$1newvalue'[/element]</td>"

but if I use

html = "<td>[element]elementreference='oldvalue';[/element]</td>";

var regex = new Regex(@"(['""#(=])" + elementReference.Key + @"(['""#)];|&)", RegexOptions.IgnoreCase);
foreach (Match match in regex.Matches(html))
{
    html = html.Replace(match.Value, match.Groups[1] + elementReference.Value + match.Groups[2]);
}

the result is

"<td>[element]elementreference='newvalue'[/element]</td>"

which is what I expected.

Can anyone explain why using Regex.Replace() did not work?

EDIT

I am not attempting to replace the inner html, I am attempting to replace the 'oldvalue' part of [element]elementreference='oldvalue'[/element], which just happens to be in a html tag. My problem lies with the fact that I am trying to replace the apostrophe around the text, by using a back reference. This apostrophe could be a number of values, that is why I am using a back reference.

http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags/1732454#1732454 — CassOnMars, Mar 01 '13 at 15:42
Don't use regex's to parse HTML - get HTMLAgilityPack instead. http://stackoverflow.com/a/1732454/847383 — PhonicUK, Mar 01 '13 at 15:43
could you further explain your problem? your provided output is exactly what I would expect your regex to create. — Sam I am says Reinstate Monica, Mar 01 '13 at 15:44
I think you should either use the available .net document classes for xml to modify xhtml - or - if needed on the client side, jQuery. — Matt, Mar 01 '13 at 15:46
also, that's not your exact code. the code you've posted won't compile — Sam I am says Reinstate Monica, Mar 01 '13 at 15:51
The regex itself works as expected. It's when I try to replace using back references that the problem occurs — Lisa Young, Mar 01 '13 at 15:52
The question http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags does not answer my back referencing problem — Lisa Young, Mar 01 '13 at 15:58
@LisaYoung is it your actual code copied and pasted, or did you just fix the error here? — Sam I am says Reinstate Monica, Mar 01 '13 at 15:58
The html is read from a file so I've just added a line from it — Lisa Young, Mar 01 '13 at 16:04

Guffa · Accepted Answer · 2013-03-01T16:53:44.057

If I try your codes, neither does any replacement, because there is no semicolon after the value that you are trying to replace.

If you remove the semicolon from the regular expression, both works:

html = Regex.Replace(html, @"(['""#(=])" + "oldvalue" + @"(['""#)]|&)", "$1" + "asdf" + "$2", RegexOptions.IgnoreCase);

does the same as:

var regex = new Regex(@"(['""#(=])" + "oldvalue" + @"(['""#)]|&)", RegexOptions.IgnoreCase);
foreach (Match match in regex.Matches(html))
{
    html = html.Replace(match.Value, match.Groups[1] + "asdf" + match.Groups[2]);
}

Edit:

When I try the updated code from the question, it works fine:

string html;
KeyValuePair<string, string> elementReference = new KeyValuePair<string, string>("oldvalue", "newvalue");

html = "<td>[element]elementreference='oldvalue';[/element]</td>";

html = Regex.Replace(html, @"(['""#(=])" + elementReference.Key + @"(['""#)];|&)", "$1" + elementReference.Value + "$2", RegexOptions.IgnoreCase);

Console.WriteLine(html);

html = "<td>[element]elementreference='oldvalue';[/element]</td>";

var regex = new Regex(@"(['""#(=])" + elementReference.Key + @"(['""#)];|&)", RegexOptions.IgnoreCase);
foreach (Match match in regex.Matches(html)) {
  html = html.Replace(match.Value, match.Groups[1] + elementReference.Value + match.Groups[2]);
}

Console.WriteLine(html);

Output:

<td>[element]elementreference='newvalue';[/element]</td>
<td>[element]elementreference='newvalue';[/element]</td>

I missed the semi-colon when editing the line in my question. The regex needs the semi-colon — Lisa Young, Mar 01 '13 at 16:06
@LisaYoung: If you add the semicolon in the original value, then the code works just fine as it is. — Guffa, Mar 01 '13 at 16:18
Every time I've run the code, using Regex.Replace() the output does contains $1 instead of the back reference. — Lisa Young, Mar 01 '13 at 16:38
@LisaYoung: I can't repeat that. When I try the code, pasted directly from the question with no modifiations at all, it works fine. — Guffa, Mar 01 '13 at 16:50

Regex back references in Regex.Replace()

1 Answers1

Edit:

Linked