Code

See regex in use here

^(?=\d+ \d+$)[\d ]{10}$

Results

Input

1 23456789
12 3456789
123 456789
1234 56789
12345 6789
123456 789
1234567 89
12345678 9

123456789
123456789 
 123456789
1 2 3456789
1 234567890

Output

Only matches are below.

1 23456789
12 3456789
123 456789
1234 56789
12345 6789
123456 789
1234567 89
12345678 9

Explanation

• ^ Assert position at the start of the line
• (?=\d+ \d+$) Positive lookahead ensuring what follows matches the following
  • \d+ Match one or more digits
  • Match a literal space
  • \d+ Match one or more digits
  • $ Assert position at the end of the line
• [\d ]{10} Match any digit or space character exactly 10 times
• $ Assert position at the end of the line

Don't use regex.

1. Check the length is 10.

2. Count the numbers and spaces:

   int nSpaces = 0, nNumbers = 0;
   for (int i = 0; i < str.length(); ++i) {
     char c = str.charAt(i);
     if (c == ' ') nSpaces++;
     else if (c >= '0' && c <= '9') nNumbers++;
   }
   

3. Check that there are 9 numbers and 1 space.

   if (nSpaces == 1 && nNumbers == 9) { ... }