Your requirements are a bit vague, even after reading all the comments. Given your example and explanations, I assume your requirements are the following:
- The input is a string consisting of (x)html tags. Your example doesn't contain this, but I assume the input can contain text between the tags.
- In the context of your problem, we do not care about nesting. So the input is really only text intermingled with tags, where opening, closing and self-closing tags are all considered equivalent.
- Tags can contain quoted values.
- You want to truncate your string such that the string is not truncated in the middle of a tag. So in the truncated string every '<' character must have a corresponding '>' character.
I'll give you two solutions, a simple one which may not be correct, depending on what the input looks like exactly, and a more complex one which is correct.
First solution
For the first solution, we first find the last '>' character before the truncate size (this corresponds to the last tag which was completely closed). After this character may come text which does not belong to any tag, so we then search for the first '<' character after the last closed tag. In code:
public static String truncate1(String input, int size)
{
if (input.length() < size) return input;
int pos = input.lastIndexOf('>', size);
int pos2 = input.indexOf('<', pos);
if (pos2 < 0 || pos2 >= size) {
return input.substring(0, size);
}
else {
return input.substring(0, pos2);
}
}
Of course this solution does not consider the quoted value strings: the '<' and '>' characters might occur inside a string, in which case they should be ignored. I mention the solution anyway because you mention your input is sanatized, so possibly you can ensure that the quoted strings never contain '<' and '>' characters.
Second solution
To consider the quoted strings, we cannot rely on standard Java classes anymore, but we have to scan the input ourselves and remember if we are currently inside a tag and inside a string or not. If we encounter a '<' character outside of a string, we remember its position, so that when we reach the truncate point we know the position of the last opened tag. If that tag wasn't closed, we truncate before the beginning of that tag. In code:
public static String truncate2(String input, int size)
{
if (input.length() < size) return input;
int lastTagStart = 0;
boolean inString = false;
boolean inTag = false;
for (int pos = 0; pos < size; pos++) {
switch (input.charAt(pos)) {
case '<':
if (!inString && !inTag) {
lastTagStart = pos;
inTag = true;
}
break;
case '>':
if (!inString) inTag = false;
break;
case '\"':
if (inTag) inString = !inString;
break;
}
}
if (!inTag) lastTagStart = size;
return input.substring(0, lastTagStart);
}
