HashtagEditText

public class HashtagEditText extends EditText implements TextWatcher {

    private boolean isAddedListener = false;

    public HashtagEditText(Context context) {
        super(context);
        registerListener();
    }

    public HashtagEditText(Context context, AttributeSet attrs) {
        super(context, attrs);
        registerListener();
    }

    public HashtagEditText(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        registerListener();
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public HashtagEditText(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        registerListener();
    }

    private void registerListener() {
        if (!isAddedListener) {
            isAddedListener = true;
            addTextChangedListener(this);
        }
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }

    @Override
    public void afterTextChanged(Editable s) {
        LinkifyUtils.resetSpans(s, s.length());
        LinkifyUtils.setSpans(s, LinkifyUtils.PATTERN_HASHTAG);
    }

    @Override
    public void onTextChanged(CharSequence _text, int start, int before, int count) {}
}

LinkifyUtils

public class LinkifyUtils {

    public static final String REGEXP_HASHTAG = "#\\w{1,120}";

    public static final Pattern PATTERN_HASHTAG = Pattern.compile(REGEXP_HASHTAG);

  

    public static void setSpans(Editable _text, Pattern pattern){
        Matcher matcher = pattern.matcher(_text.toString());
        while (matcher.find()) {
            int start = matcher.start();
            int end = matcher.end();

            ForegroundColorSpan fcs = new ForegroundColorSpan(Color.RED);
            if (_text.getSpans(start, end, ForegroundColorSpan.class).length == 0) {
                _text.setSpan(fcs, start, end, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
            }
        }
    }

    public static void resetSpans(Editable _text, int _length) {
        ForegroundColorSpan[] foregroundSpans =
                _text.getSpans(0, _length, ForegroundColorSpan.class);
        for (ForegroundColorSpan spanToRemove : foregroundSpans) {
            _text.removeSpan(spanToRemove);
        }
    }
}