I got these warnings with Kotlin + Proguard:

Warning: kotlin.internal.JRE8PlatformImplementations: can't find referenced method 'int start(java.lang.String)' in library class java.util.regex.Matcher
Warning: kotlin.internal.JRE8PlatformImplementations: can't find referenced method 'int end(java.lang.String)' in library class java.util.regex.Matcher
Warning: kotlin.internal.JRE8PlatformImplementations: can't find referenced method 'java.lang.String group(java.lang.String)' in library class java.util.regex.Matcher

It seems that this proguard instruction doesn't help:

-keep class java.util.regex.Matcher { *; }

@rockerhieu It seems that you're passing path to JDK7 or less as a library to proguard instead of JDK8.