Regex for converting CamelCase to camel_case in java

You can use below code snippet:

String replaceAll = key.replaceAll("(.)(\\p{Upper})", "$1_$2").toLowerCase();

I can't provide RegEx, it would be insanely complex anyway.

Try this function with automatic recognition of acronyms.

Unfortunately Guava lib doesn't auto detect upper case acronyms, so "bigCAT" would be converted to "BIG_C_A_T"

/**
 * Convert to UPPER_UNDERSCORE format detecting upper case acronyms
 */
private String upperUnderscoreWithAcronyms(String name) {
    StringBuffer result = new StringBuffer();
    boolean begin = true;
    boolean lastUppercase = false;
    for( int i=0; i < name.length(); i++ ) {
        char ch = name.charAt(i);
        if( Character.isUpperCase(ch) ) {
            // is start?
            if( begin ) {
                result.append(ch);
            } else {
                if( lastUppercase ) {
                    // test if end of acronym
                    if( i+1<name.length() ) {
                        char next = name.charAt(i+1);
                        if( Character.isUpperCase(next) ) {
                            // acronym continues
                            result.append(ch);
                        } else {
                            // end of acronym
                            result.append('_').append(ch);
                        }
                    } else {
                        // acronym continues
                        result.append(ch);
                    }
                } else {
                    // last was lowercase, insert _
                    result.append('_').append(ch);
                }
            }
            lastUppercase=true;
        } else {
            result.append(Character.toUpperCase(ch));
            lastUppercase=false;
        }
        begin=false;
    }
    return result.toString();
}

bind the lower case and upper case as two group,it will be ok

public  class Main
{
    public static void main(String args[])
    {
        String regex = "([a-z])([A-Z]+)";
        String replacement = "$1_$2";
        System.out.println("CamelCaseToSomethingElse"
                           .replaceAll(regex, replacement)
                           .toLowerCase());
    }
}

See this question and CaseFormat from guava

in your case, something like:

CaseFormat.UPPER_CAMEL.to(CaseFormat.LOWER_UNDERSCORE, "SomeInput");