String#strip!

static VALUE
rb_str_strip_bang(int argc, VALUE *argv, VALUE str)
{
    char *start;
    long olen, loffset, roffset;
    rb_encoding *enc;

    str_modify_keep_cr(str);
    enc = STR_ENC_GET(str);
    RSTRING_GETMEM(str, start, olen);

    if (argc > 0) {
        char table[TR_TABLE_SIZE];
        VALUE del = 0, nodel = 0;

        tr_setup_table_multi(table, &del, &nodel, str, argc, argv);
        loffset = lstrip_offset_table(str, start, start+olen, enc, table, del, nodel);
        roffset = rstrip_offset_table(str, start+loffset, start+olen, enc, table, del, nodel);
    }
    else {
        loffset = lstrip_offset(str, start, start+olen, enc);
        roffset = rstrip_offset(str, start+loffset, start+olen, enc);
    }

    if (loffset > 0 || roffset > 0) {
        long len = olen-roffset;
        if (loffset > 0) {
            len -= loffset;
            memmove(start, start + loffset, len);
        }
        STR_SET_LEN(str, len);
        TERM_FILL(start+len, rb_enc_mbminlen(enc));
        return str;
    }
    return Qnil;
}