$cat TestRegex.java

    import java.util.regex.*;
    class TestRegex {
       public static void main( String ... args ) {
           System.out.printf("%s ~= %s ? %s  %n" , args[0].replaceAll("\\","\\\\"), args[1], Pattern.matches( args[0], args[1] ) );
       }
    }


$javac TestRegex.java
$java TestRegex "\d+" "1"
Exception in thread "main" java.util.regex.PatternSyntaxException: Unexpected internal error near index 1
\
 ^
    at java.util.regex.Pattern.error(Pattern.java:1713)
    at java.util.regex.Pattern.compile(Pattern.java:1466)
    at java.util.regex.Pattern.<init>(Pattern.java:1133)
    at java.util.regex.Pattern.compile(Pattern.java:823)
    at java.lang.String.replaceAll(String.java:2189)
    at TestRegex.main(TestRegex.java:5)