LenientUriConverter.java

import java.net.URI;
import java.net.URLEncoder;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class LenientUriConverter {

  final static Pattern URL_PATTERN = Pattern.compile("(.+?://.+?)(/+.*?)?([?#].+)?");
  // NOTE: /+ normalizes any double slashes in the path
  final static Pattern PATHS_PATTERN = Pattern.compile("/+([^/]+)");

  public URI convert(String s) throws Exception {
    final Matcher m = URL_PATTERN.matcher(s);
    if (!m.matches()) {
      throw new IllegalArgumentException("Failed to parse url: " + s);
    }

    StringBuilder sb = new StringBuilder(m.group(1));
    final String pathsPart = m.group(2);
    if (pathsPart != null) {
      final Matcher pathsMatcher = PATHS_PATTERN.matcher(pathsPart);
      while (pathsMatcher.find()) {
        final String content = pathsMatcher.group(1);
        sb.append("/")
            .append(URLEncoder.encode(
                // flip back any pre-encoded spaces
                content.replace('+', ' '),
                "utf-8"
            ));
      }
    }
    final String trailer = m.group(3);
    if (trailer != null) {
      sb.append(trailer);
    }

    return URI.create(sb.toString());
  }
}

Needed this to handle mod URLs from CurseForge such as:

• https://media.forgecdn.net/files/3482/169/Valhelsia+3-3.4.4-SERVER.zip
• https://files.forgecdn.net/files/2320/259/[1.10.x]FenceOverhaul-1.2.1.jar

where java.net.URI itself did not like the square brackets and would re-encode the +s