Landline number + area code + extension number regular matching rules

　Note: Code Verify 3-4 digits, 7-8 intermediate landline digit number verification, verification rear extension of 1-5 digits. 
　regFun () { 
      the let Phone = 021-88888999-1234; // number you entered 
      let regexp = / ^ ((\ d {3,4} \ -) |) \ d {7,8} (| ([ - \ u8f6c]. 1} {\ D {l, 5})) $ /;   // regular 
      IF (regexp.test (Phone)) { 
        the console.log (Phone + '--- verified' ) 
      } the else { 
        Console .log (Phone + '--- authentication failed' ) 
      } 
    }