jodd-core/src/main/java/jodd/util/StringUtil.java
public static String insert(final String src, final String insert, int offset) {
		if (offset < 0) {
			offset = 0;
		}
		if (offset > src.length()) {
			offset = src.length();
		}
		StringBuilder sb = new StringBuilder(src);
		sb.insert(offset, insert);
		return sb.toString();
	}
public static String insert(final String src, final String insert, int offset) {
		if (offset < 0) {
			offset = 0;
		}
		if (offset > src.length()) {
			offset = src.length();
		}
		StringBuilder sb = new StringBuilder(src);
		sb.insert(offset, insert);
		return sb.toString();
	}
Inserts a string on provided offset.