Why is a left parenthesis being escaped in this Regex?

Question

I'm using an HTML sanitizing whitelist code found here: http://refactormycode.com/codes/333-sanitize-html I needed to add the "font" tag as an additional tag to match, so I tried adding this condition after the ` tag checking // Non-escaped expression (for testing in a Regex editor app) // ^$ if (!IsMatch(tagname, @"")) { html = html.Remove(tag.Index, tag.Length); } } ``` Aside from the condition above, my code is almost identical to the code in the page I linked to. When I try to test this in C#, it throws an exception saying "`Not enough )'s`". I've counted the parenthesis severa…

Accepted Answer

Your IsMatch Method is using the option `RegexOptions.IgnorePatternWhitespace`, that allows you to put comments inside the regular expressions, so you have to scape the # chatacter, otherwise it will be interpreted as a comment.

```
if (!IsMatch(tagname,@"<font(\s*size=""\d{1}"")?
    (\s*color=""((\#[0-9a-f]{6})|(\#[0-9a-f]{3})|red|green|blue|black|white)"")?
    (\s*face=""(Arial|Courier New|Garamond|Georgia|Tahoma|Verdana)"")?
    \s?>"))
{
    html = html.Remove(tag.Index, tag.Length);
}
```

Why is a left parenthesis being escaped in this Regex?

Solution

Problem