How to add dot before the first letter in a string?

Consider this string s = " GOTO ok1".
How to add a efficient if check whether there exists only a single dot before a letter and after a space.
If dot doesn't exists, I want to add a dot before the first letter.
I want the string to be this s = " .GOTO ok1". Notice it still holds the leading space.

How to efficiently do this in java(using regex)/ groovy (using closures)?
Does a one or two liner code exists for this ?

In Java you could try something like this:

Pattern.compile("^(\\s*)(?:\\.)*(\\w)").matcher(input).replaceFirst("$1.$2");

It would produce following outputs:

" GOTO ok1"    -> " .GOTO ok1",
" .GOTO ok1"   -> " .GOTO ok1",
"GOTO ok1"     -> ".GOTO ok1",
"  ..GOTO ok1" -> "  .GOTO ok1",
"       "      -> "       "

Matcher.replaceFirst replaces first occurrence of the pattern with the provided replacement. $1 and $2 in the replacement string are references to capture-groups within the pattern:

given string " ....GOTO ok1":

• full pattern match: " ....G"
• group 1: " " (leading spaces)
• group 2: "G" (first letter)
• thus, " ....G" is replaced with " .G"

In practice you would rather want to compile the pattern once and reuse it:

class RegexTest {

    private final static Pattern PATTERN = Pattern.compile("^(\\s*)(?:\\.)*(\\w)");

    @Test
    void test() {
        var examples = Map.of(
            " GOTO ok1",  " .GOTO ok1",
            " .GOTO ok1", " .GOTO ok1",
            "GOTO ok1", ".GOTO ok1",
            "      ..GOTO ok1", "      .GOTO ok1",
            "       ", "       "
        );
        examples.forEach((input, expected) -> {
            assertEquals(expected, PATTERN.matcher(input).replaceFirst("$1.$2"));
        });
    }

}

Performance-wise, regular expressions are pretty expensive, the most effective would be a simple loop:

String format(String in) {
    StringBuilder builder = new StringBuilder();
    for(int i=0; i<in.length(); i++) {
        switch (in.charAt(i)) {
            case ' ':
                builder.append(' ');
                break;
            case '.':
                break;
            default:
                builder.append(".").append(in.substring(i));
                return builder.toString();
        }
    }
    return builder.toString();
}

If you are looking to process a large text looking for these occurrences i don't think its possible in 2 lines of code.Hope this helps or gives you some ideas:

    String  d = " GOTO ok1";
    String  b = " .";
    StringBuilder builder = new StringBuilder(d); 
    builder.deleteCharAt(0);
    System.out.println(b+builder); // Does the job in your example but prob not what you are looking for
    
    String s = " .GOTO ok1";
    String count = s.trim();
    System.out.println(count); // This will only get rid of spaces
    
    int n = 2;
    String upToNCharacters = s.substring(0, Math.min(s.length(), n));
    int x = upToNCharacters.indexOf('.');
    // Here you can add IF x is 0 or 1 then "." exists
    // But it is not 2 lines of code as you requested
    
    System.out.println(x);