Tested and working, but YMMV. Posted to CFLib.org but not up there yet.

function getLeadingHtml(input, maxChars) {
	// token matches a word, tag, or special character
	var	token = "[[:word:]]+|[^[:word:]< ]|(?:]*(\/)?>)|< ";
	var	selfClosingTag = "^(?:[hb]r|img)$";
	var	output = "";
	var	charCount = 0;
	var	openTags = ""; var strPos = 0; var tag = "";
	var i = 1;

	var	match = REFind(token, input, i, "true");

	while ( (charCount LT maxChars) AND match.pos[1] ) {
		// If this is an HTML tag
		if (match.pos[3]) {
			output = output & Mid(input, match.pos[1], match.len[1]);
			tag = Mid(input, match.pos[3], match.len[3]);
			// If this is not a self-closing tag
			if ( NOT ( match.pos[4] OR REFindNoCase(selfClosingTag, tag) ) ) {
				// If this is a closing tag
				if ( match.pos[2] AND ListFindNoCase(openTags, tag) ) {
					openTags = ListDeleteAt(openTags, ListFindNoCase(openTags, tag));
				} else {
					openTags = ListAppend(openTags, tag);
				}
			}
		} else {
			charCount = charCount +  match.len[1];
			if (charCount LTE maxChars) output = output & Mid(input, match.pos[1], match.len[1]);
		}

		i = i + match.len[1];
		match = REFind(token, input, i, "true");
	}

	// Close any tags which were left open
	while ( ListLen(openTags) ) {
		output = output & "";
		openTags = ListDeleteAt(openTags, ListLen(openTags));
	}

	if ( Len(input) GT Len(output) )
		output = output & "…";

	return output;
}

token.lastIndex = 0;

before the while statement.

public string GetLeadingHtml(string input, int? maxChars) {
// token matches a word, tag, or special character
Regex token = new Regex(“\\w+|[^\\w<]|<(/)?(\\w+)[^>]*(/)?>|<”);
Regex selfClosingTag = new Regex(“^(?:[hb]r|img)$”, RegexOptions.IgnoreCase);
string output = String.Empty;
int charCount = 0;
List<string> openTags = new List<string>();
Match match;

// Set the default for the max number of characters
// (only counts characters outside of HTML tags)
if(maxChars == null)
maxChars = 250;

MatchCollection matches = token.Matches(input);
int matchCounter = 0;

// (only counts characters outside of HTML tags)
while (charCount < maxChars && matchCounter < matches.Count)
{
match = matches[matchCounter];
matchCounter++;
// If this is an HTML tag
if (match.Groups[2].Success)
{
output += match.Groups[0].Value;
// If this is not a self-closing tag
if (!(match.Groups[3].Success || selfClosingTag.IsMatch(match.Groups[2].Value)))
{
// If this is a closing tag
if (match.Groups[1].Success)
{
openTags.RemoveAt(openTags.Count – 1);
}
else
{
openTags.Add(match.Groups[2].Value);
}
}
}
else
{
charCount += match.Groups[0].Length;
if (charCount <= maxChars)
output += match.Groups[0].Value;

}
}

// Close any tags which were left open
var i = openTags.Count;
while (i– > 0) output += “</” + openTags[i] + “>”;

return output;
}