diff --git a/src/stdlib_strings_format_string.fypp b/src/stdlib_strings_format_string.fypp
index 29cda718d..708c4f6a0 100644
--- a/src/stdlib_strings_format_string.fypp
+++ b/src/stdlib_strings_format_string.fypp
@@ -29,13 +29,10 @@ contains
         !! Format ${type}$ variable as character sequence
         character(len=buffer_len) :: buffer
 
-        write(buffer, *) '('//&
-                        format_string_r${kind}$(val%re, fmt)//','// &
-                        format_string_r${kind}$(val%im, fmt)//')'
-
-        string = trim(buffer)
+        string = '('//format_string_r${kind}$(val%re, fmt)//','// &
+            format_string_r${kind}$(val%im, fmt)//')'
 
     end procedure format_string_${type[0]}$${kind}$
     #:endfor
 
-end submodule stdlib_strings_format_string
\ No newline at end of file
+end submodule stdlib_strings_format_string
diff --git a/src/tests/string/test_strings_format_string.f90 b/src/tests/string/test_strings_format_string.f90
index 0362b4258..0cfadeb9f 100644
--- a/src/tests/string/test_strings_format_string.f90
+++ b/src/tests/string/test_strings_format_string.f90
@@ -1,24 +1,82 @@
 program test_strings_format_string
-    use, non_intrinsic :: stdlib_strings, only: format_string
+    use stdlib_strings, only: format_string, starts_with
+    use stdlib_error, only: check
+    use stdlib_optval, only: optval
     implicit none
     print *, 'format_string(complex) : '
-        print *, format_string((1, 1))
-        print *, format_string((1, 1), '(F6.2)')
-        print *, format_string((1, 1), '(F6.2)'), format_string((2, 2), '(F7.3)')
+    call check_formatter(format_string((1, 1)), "(1.00000000,1.00000000)", &
+        & "Default formatter for complex number")
+    call check_formatter(format_string((1, 1), '(F6.2)'), "(  1.00,  1.00)", &
+        & "Formatter for complex number")
+    call check_formatter(format_string((-1, -1), '(F6.2)'), "( -1.00, -1.00)", &
+        & "Formatter for negative complex number")
+    call check_formatter(format_string((1, 1), '(SP,F6.2)'), "( +1.00, +1.00)", &
+        & "Formatter with sign control descriptor for complex number")
+    call check_formatter(format_string((1, 1), '(F6.2)')//format_string((2, 2), '(F7.3)'), &
+        & "(  1.00,  1.00)(  2.000,  2.000)", &
+        & "Multiple formatters for complex numbers")
     print *, 'format_string(integer) : '
-        print *, format_string(100)
-        print *, format_string(100, '(I6)')
-        print *, format_string(100, '(I6)'), format_string(1000, '(I7)')
+    call check_formatter(format_string(100), "100", &
+        & "Default formatter for integer number")
+    call check_formatter(format_string(100, '(I6)'), "   100", &
+        & "Formatter for integer number")
+    call check_formatter(format_string(100, '(I0.6)'), "000100", &
+        & "Formatter with zero padding for integer number")
+    call check_formatter(format_string(100, '(I6)')//format_string(1000, '(I7)'), &
+        & "   100   1000", &
+        & "Multiple formatters for integers")
+    call check_formatter(format_string(34, '(B8)'), "  100010", &
+        & "Binary formatter for integer number")
+    call check_formatter(format_string(34, '(O0.3)'), "042", &
+        & "Octal formatter with zero padding for integer number")
+    call check_formatter(format_string(34, '(Z3)'), " 22", &
+        & "Hexadecimal formatter for integer number")
     print *, 'format_string(real) : '
-        print *, format_string(100.)
-        print *, format_string(100., '(F12.2)')
-        print *, format_string(100., '(F6.2)'), &
-                    format_string(1000., '(F7.3)'), format_string(1000, '(F7.3)')
-                        !! Wrong demonstration
+    call check_formatter(format_string(100.), "100.000000", &
+        & "Default formatter for real number")
+    call check_formatter(format_string(100., '(F6.2)'), "100.00", &
+        & "Formatter for real number")
+    call check_formatter(format_string(289., '(E7.2)'), ".29E+03", &
+        & "Exponential formatter with rounding for real number")
+    call check_formatter(format_string(128., '(ES8.2)'), "1.28E+02", &
+        & "Exponential formatter for real number")
+    ! Wrong demonstration
+    call check_formatter(format_string(-100., '(F6.2)'), "*", &
+        & "Too narrow formatter for signed real number", partial=.true.)
+    call check_formatter(format_string(1000., '(F6.3)'), "*", &
+        & "Too narrow formatter for real number", partial=.true.)
+    call check_formatter(format_string(1000, '(F7.3)'), "*", &
+        & "Real formatter for integer number", partial=.true.)
     print *, 'format_string(logical) : '
-        print *, format_string(.true.)
-        print *, format_string(.true., '(L2)')
-        print *, format_string(.false., '(L2)'), format_string(.true., '(L5)'), &
-                    format_string(.false., '(I5)')
-                        !! Wrong demonstration
-end program test_strings_format_string
\ No newline at end of file
+    call check_formatter(format_string(.true.), "T", &
+        & "Default formatter for logcal value")
+    call check_formatter(format_string(.true., '(L2)'), " T", &
+        & "Formatter for logical value")
+    call check_formatter(format_string(.false., '(L2)')//format_string(.true., '(L5)'), &
+        & " F    T", &
+        & "Multiple formatters for logical values")
+    ! Wrong demonstration
+    call check_formatter(format_string(.false., '(I5)'), "*", &
+        & "Integer formatter for logical value", partial=.true.)
+
+contains
+    subroutine check_formatter(actual, expected, description, partial)
+        character(len=*), intent(in) :: actual, expected, description
+        logical, intent(in), optional :: partial
+        logical :: stat
+        character(len=:), allocatable :: msg
+
+        if (optval(partial, .false.)) then
+            stat = starts_with(actual, expected)
+        else
+            stat = actual == expected
+        end if
+        if (.not.stat) then
+            msg = description // new_line("a") // &
+                & "Expected: '"//expected//"' but got '"//actual//"'"
+        else
+            print '(" - ", a, /, "   Result: ''", a, "''")', description, actual
+        end if
+        call check(stat, msg)
+    end subroutine check_formatter
+end program test_strings_format_string