private EditText mMacEdit = (EditText)findViewById(R.id.edit_mac);

/**
 * Registers TextWatcher for MAC EditText field. Automatically adds colons,
 * switches the MAC to upper case and handles the cursor position.
 */
private void registerAfterMacTextChangedCallback() {
    mMacEdit.addTextChangedListener(new TextWatcher() {
        String mPreviousMac = null;

        /* (non-Javadoc)
         * Does nothing.
         * @see android.text.TextWatcher#afterTextChanged(android.text.Editable)
         */
        @Override
        public void afterTextChanged(Editable arg0) {
        }

        /* (non-Javadoc)
         * Does nothing.
         * @see android.text.TextWatcher#beforeTextChanged(java.lang.CharSequence, int, int, int)
         */
        @Override
        public void beforeTextChanged(CharSequence arg0, int arg1, int arg2, int arg3) {
        }

        /* (non-Javadoc)
         * Formats the MAC address and handles the cursor position.
         * @see android.text.TextWatcher#onTextChanged(java.lang.CharSequence, int, int, int)
         */
        @SuppressLint("DefaultLocale")
        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            String enteredMac = mMacEdit.getText().toString().toUpperCase();
            String cleanMac = clearNonMacCharacters(enteredMac);
            String formattedMac = formatMacAddress(cleanMac);

            int selectionStart = mMacEdit.getSelectionStart();
            formattedMac = handleColonDeletion(enteredMac, formattedMac, selectionStart);
            int lengthDiff = formattedMac.length() - enteredMac.length();

            setMacEdit(cleanMac, formattedMac, selectionStart, lengthDiff);
        }

        /**
         * Strips all characters from a string except A-F and 0-9.
         * @param mac       User input string.
         * @return          String containing MAC-allowed characters.
         */
        private String clearNonMacCharacters(String mac) {
            return mac.toString().replaceAll("[^A-Fa-f0-9]", "");
        }

        /**
         * Adds a colon character to an unformatted MAC address after
         * every second character (strips full MAC trailing colon)
         * @param cleanMac      Unformatted MAC address.
         * @return              Properly formatted MAC address.
         */
        private String formatMacAddress(String cleanMac) {
            int grouppedCharacters = 0;
            String formattedMac = "";

            for (int i = 0; i < cleanMac.length(); ++i) {
                formattedMac += cleanMac.charAt(i);
                ++grouppedCharacters;

                if (grouppedCharacters == 2) {
                    formattedMac += ":";
                    grouppedCharacters = 0;
                }
            }

            // Removes trailing colon for complete MAC address
            if (cleanMac.length() == 12)
                formattedMac = formattedMac.substring(0, formattedMac.length() - 1);

            return formattedMac;
        }

        /**
         * Upon users colon deletion, deletes MAC character preceding deleted colon as well.
         * @param enteredMac            User input MAC.
         * @param formattedMac          Formatted MAC address.
         * @param selectionStart        MAC EditText field cursor position.
         * @return                      Formatted MAC address.
         */
        private String handleColonDeletion(String enteredMac, String formattedMac, int selectionStart) {
            if (mPreviousMac != null && mPreviousMac.length() > 1) {
                int previousColonCount = colonCount(mPreviousMac);
                int currentColonCount = colonCount(enteredMac);

                if (currentColonCount < previousColonCount) {
                    formattedMac = formattedMac.substring(0, selectionStart - 1) + formattedMac.substring(selectionStart);
                    String cleanMac = clearNonMacCharacters(formattedMac);
                    formattedMac = formatMacAddress(cleanMac);
                }
            }
            return formattedMac;
        }

        /**
         * Gets MAC address current colon count.
         * @param formattedMac      Formatted MAC address.
         * @return                  Current number of colons in MAC address.
         */
        private int colonCount(String formattedMac) {
            return formattedMac.replaceAll("[^:]", "").length();
        }

        /**
         * Removes TextChange listener, sets MAC EditText field value,
         * sets new cursor position and re-initiates the listener.
         * @param cleanMac          Clean MAC address.
         * @param formattedMac      Formatted MAC address.
         * @param selectionStart    MAC EditText field cursor position.
         * @param lengthDiff        Formatted/Entered MAC number of characters difference.
         */
        private void setMacEdit(String cleanMac, String formattedMac, int selectionStart, int lengthDiff) {
            mMacEdit.removeTextChangedListener(this);
            if (cleanMac.length() <= 12) {
                mMacEdit.setText(formattedMac);
                mMacEdit.setSelection(selectionStart + lengthDiff);
                mPreviousMac = formattedMac;
            } else {
                mMacEdit.setText(mPreviousMac);
                mMacEdit.setSelection(mPreviousMac.length());
            }
            mMacEdit.addTextChangedListener(this);
        }
    });
}