TextAlignment on TextView for Rtl and Ltr text

Question

How do TextView decides to align text left or right?

If you want to align a piece of text, you should apply AlignmentSpan which should be one of these 3 values(NORMAL, OPPOSITE, CENTER).

If you set alignment to NORMAL depends on what is first character it decides to align text left or right.

Below are some examples of text and alignment:

Hi : Left

@Hi : Left

سلام : Right

@سلام : Right

*@ Hi : Left

*@ سلام : Right

Hi سلام : Left

سلام Hi : Right

I wanna know how exactly textView determines text alignment? If possible please give a regex pattern to determine text alignment.

I want something like this:

Pattern regex = Pattern.compile(patternString);
if(regex.matcher(text).matches()) {
    // now text will be left aligned
}

I wanna find out patternString value.

Answer 1

It seems the algorithm decides based the on first letter-based character of the text (i.e. it finds the non-symbolic character and check if it is an RTL character or not).

You can use \p{L} or \p{Letter} in RegEx to find the first match, then compare its first character with the range of RTL unicode characters:

boolean isRtl(String str) {
    Pattern p = Pattern.compile("([^\\p{L}]*)(\\p{L})(.*)";
    Matcher m = p.matcher(str);
    if (m.matches()) {
      String firstMatch = m.group(2);
      char c = firstMatch.charAt(0);
      return c >= 0x590 && c <= 0x6ff;
    }
    return false;
}