Fix parsing of regexps that toggle extended mode on/off inside regexp

This was broken in ec3542229b. That commit
didn't handle cases where extended mode was turned on/off inside the
regexp.  There are two ways to turn extended mode on/off:

```
/(?-x:#y)#z
/x =~ '#y'

/(?-x)#y(?x)#z
/x =~ '#y'
```

These can be nested inside the same regexp:

```
/(?-x:(?x)#x
(?-x)#y)#z
/x =~ '#y'
```

As you can probably imagine, this makes handling these regexps
somewhat complex. Due to the nesting inside portions of regexps,
the unassign_nonascii function needs to be recursive.  In
recursive mode, it needs to track both opening and closing
parentheses, similar to how it already tracked opening and
closing brackets for character classes.

When scanning the regexp and coming to `(?` not followed by `#`,
scan for options, and use `x` and `i` to determine whether to
turn on or off extended mode.  For `:`, indicting only the
current regexp section should have the extended mode
switched, recurse with the extended mode set or unset. For `)`,
indicating the remainder of the regexp (or current regexp portion
if already recursing) should turn extended mode on or off, just
change the extended mode flag and keep scanning.

While testing this, I noticed that `a`, `d`, and `u` are accepted
as options, in addition to `i`, `m`, and `x`, but I can't see
where those options are documented.  I'm not sure whether or not
handling  `a`, `d`, and `u` as options is a bug.

Fixes [Bug #19379]

re.c

@@ -2801,14 +2801,18 @@ unescape_unicode_bmp(const char **pp, const char *end,
 }
 
 static int
-unescape_nonascii(const char *p, const char *end, rb_encoding *enc,
+unescape_nonascii0(const char **pp, const char *end, rb_encoding *enc,
         VALUE buf, rb_encoding **encp, int *has_property,
-        onig_errmsg_buffer err, int options)
+        onig_errmsg_buffer err, int options, int recurse)
 {
+    const char *p = *pp;
     unsigned char c;
     char smallbuf[2];
     int in_char_class = 0;
+    int parens = 1; /* ignored unless recurse is true */
+    int extended_mode = options & ONIG_OPTION_EXTEND;
 
+begin_scan:
     while (p < end) {
         int chlen = rb_enc_precise_mbclen(p, end, enc);
         if (!MBCLEN_CHARFOUND_P(chlen)) {
@@ -2920,7 +2924,7 @@ escape_asis:
             break;
 
           case '#':
-            if ((options & ONIG_OPTION_EXTEND) && !in_char_class) {
+            if (extended_mode && !in_char_class) {
                 /* consume and ignore comment in extended regexp */
                 while ((p < end) && ((c = *p++) != '\n'));
                 break;
@@ -2937,51 +2941,134 @@ escape_asis:
             }
             rb_str_buf_cat(buf, (char *)&c, 1);
             break;
-          case '(':
+          case ')':
-            if (!in_char_class && p + 1 < end && *p == '?' && *(p+1) == '#') {
+            rb_str_buf_cat(buf, (char *)&c, 1);
-                /* (?# is comment inside any regexp, and content inside should be ignored */
+            if (!in_char_class && recurse) {
-                const char *orig_p = p;
+                if (--parens == 0) {
-                int cont = 1;
+                    *pp = p;
-
+                    return 0;
-                while (cont && (p < end)) {
-                    switch (c = *p++) {
-                      default:
-                        if (!(c & 0x80)) break;
-                        --p;
-                        /* fallthrough */
-                      case '\\':
-                        chlen = rb_enc_precise_mbclen(p, end, enc);
-                        if (!MBCLEN_CHARFOUND_P(chlen)) {
-                            goto invalid_multibyte;
-                        }
-                        p += MBCLEN_CHARFOUND_LEN(chlen);
-                        break;
-                      case ')':
-                        cont = 0;
-                        break;
-                    }
                 }
-
-                if (cont) {
-                    /* unterminated (?#, rewind so it is syntax error */
-                    p = orig_p;
-                    c = '(';
-                    rb_str_buf_cat(buf, (char *)&c, 1);
-                }
-            }
-            else {
-                rb_str_buf_cat(buf, (char *)&c, 1);
             }
             break;
+          case '(':
+            if (!in_char_class && p + 1 < end && *p == '?') {
+                if (*(p+1) == '#') {
+                    /* (?# is comment inside any regexp, and content inside should be ignored */
+                    const char *orig_p = p;
+                    int cont = 1;
+
+                    while (cont && (p < end)) {
+                        switch (c = *p++) {
+                          default:
+                            if (!(c & 0x80)) break;
+                            --p;
+                            /* fallthrough */
+                          case '\\':
+                            chlen = rb_enc_precise_mbclen(p, end, enc);
+                            if (!MBCLEN_CHARFOUND_P(chlen)) {
+                                goto invalid_multibyte;
+                            }
+                            p += MBCLEN_CHARFOUND_LEN(chlen);
+                            break;
+                          case ')':
+                            cont = 0;
+                            break;
+                        }
+                    }
+
+                    if (cont) {
+                        /* unterminated (?#, rewind so it is syntax error */
+                        p = orig_p;
+                        c = '(';
+                        rb_str_buf_cat(buf, (char *)&c, 1);
+                    }
+                    break;
+                } else {
+                    /* potential change of extended option */
+                    int invert = 0;
+                    int local_extend = 0;
+                    const char *s;
+
+                    if (recurse) {
+                        parens++;
+                    }
+
+                    for(s = p+1; s < end; s++) {
+                        switch(*s) {
+                            case 'x':
+                                local_extend = invert ? -1 : 1;
+                                break;
+                            case '-':
+                                invert = 1;
+                                break;
+                            case ':':
+                            case ')':
+                                if (local_extend == 0 ||
+                                    (local_extend == -1 && !extended_mode) ||
+                                    (local_extend == 1 && extended_mode)) {
+                                    /* no changes to extended flag */
+                                    goto fallthrough;
+                                }
+
+                                if (*s == ':') {
+                                    /* change extended flag until ')' */
+                                    int local_options = options;
+                                    if (local_extend == 1) {
+                                         local_options |= ONIG_OPTION_EXTEND;
+                                    } else {
+                                         local_options &= ~ONIG_OPTION_EXTEND;
+                                    }
+
+                                    rb_str_buf_cat(buf, (char *)&c, 1);
+                                    int ret = unescape_nonascii0(&p, end, enc, buf, encp,
+                                                                has_property, err,
+                                                                local_options, 1);
+                                    if (ret < 0) return ret;
+                                    goto begin_scan;
+                                } else {
+                                    /* change extended flag for rest of expression */
+                                    extended_mode = local_extend == 1;
+                                    goto fallthrough;
+                                }
+                            case 'i':
+                            case 'm':
+                            case 'a':
+                            case 'd':
+                            case 'u':
+                                /* other option flags, ignored during scanning */
+                                break;
+                            default:
+                                /* other character, no extended flag change*/
+                                goto fallthrough;
+                        }
+                    }
+                }
+            } else if (!in_char_class && recurse) {
+                parens++;
+            }
+            /* FALLTHROUGH */
           default:
+fallthrough:
             rb_str_buf_cat(buf, (char *)&c, 1);
             break;
         }
     }
 
+    if (recurse) {
+        *pp = p;
+    }
     return 0;
 }
 
+static int
+unescape_nonascii(const char *p, const char *end, rb_encoding *enc,
+        VALUE buf, rb_encoding **encp, int *has_property,
+        onig_errmsg_buffer err, int options)
+{
+    return unescape_nonascii0(&p, end, enc, buf, encp, has_property,
+                              err, options, 0);
+}
+
 static VALUE
 rb_reg_preprocess(const char *p, const char *end, rb_encoding *enc,
         rb_encoding **fixed_enc, onig_errmsg_buffer err, int options)

test/ruby/test_regexp.rb

@@ -144,6 +144,62 @@ class TestRegexp < Test::Unit::TestCase
     assert_raise(SyntaxError) {eval "/# \\users/"}
   end
 
+  def test_nonextended_section_of_extended_regexp_bug_19379
+    assert_separately([], <<-'RUBY')
+      re = /(?-x:#)/x
+      assert_match(re, '#')
+      assert_not_match(re, '-')
+
+      re = /(?xi:#
+      y)/
+      assert_match(re, 'Y')
+      assert_not_match(re, '-')
+
+      re = /(?mix:#
+      y)/
+      assert_match(re, 'Y')
+      assert_not_match(re, '-')
+
+      re = /(?x-im:#
+      y)/i
+      assert_match(re, 'y')
+      assert_not_match(re, 'Y')
+
+      re = /(?-imx:(?xim:#
+      y))/x
+      assert_match(re, 'y')
+      assert_not_match(re, '-')
+
+      re = /(?x)#
+      y/
+      assert_match(re, 'y')
+      assert_not_match(re, 'Y')
+
+      re = /(?mx-i)#
+      y/i
+      assert_match(re, 'y')
+      assert_not_match(re, 'Y')
+
+      re = /(?-imx:(?xim:#
+      (?-x)y#))/x
+      assert_match(re, 'Y#')
+      assert_not_match(re, '-#')
+
+      re = /(?imx:#
+      (?-xim:#(?im)#(?x)#
+      )#
+      (?x)#
+      y)/
+      assert_match(re, '###Y')
+      assert_not_match(re, '###-')
+
+      re = %r{#c-\w+/comment/[\w-]+}
+      re = %r{https?://[^/]+#{re}}x
+      assert_match(re, 'http://foo#c-x/comment/bar')
+      assert_not_match(re, 'http://foo#cx/comment/bar')
+    RUBY
+  end
+
   def test_union
     assert_equal :ok, begin
       Regexp.union(