foo

$ gcc testonig.c -I /usr/local/include/ -L /usr/local/lib/ -llibonigmo && ./a.exe
PATTERN:     '(?<!ss)'
STRING:      'aaaa'

PATTERN: /(?<!ss)/ (UTF-8)
<list:600000460>
   <anchor:6000486f0> look_behind_not
      <list:600048770>
         <alt:6000487b0>
            <string:6000487f0>s
            <string:600048830>S
            <string:6000488b0> 0xc5 0xbf
         <string:600048a30>s
   <anchor:600048730> look_behind_not
      <string:600048930> 0xc3 0x9f
   <anchor:600048ab0> look_behind_not
      <string:6000489b0> 0xe1 0xba 0x9e
optimize: NONE
  anchor: []