kc3-lang/md4c

diff --git a/README.md b/README.md
index e7c0157..556a6ea 100644
--- a/README.md
+++ b/README.md
@@ -140,7 +140,9 @@ consideration.
 
 - **Miscellaneous:**
   - [x] Permissive ATX headers: `###Header` (without space)
-  - [ ] Permissive autolinks: `http://google.com` (without `<`...`>`)
+  - [x] Permissive URL autolinks: `http://google.com` (without `<`...`>`)
+  - [ ] Permissive e-mail autolinks: `john.dow@example.com`
+        (without `<`...`>` and `mailto:`)
   - [x] Disabling indented code blocks
   - [x] Disabling raw HTML blocks/spans
 
diff --git a/md2html/md2html.c b/md2html/md2html.c
index 7368b4e..eaf0371 100644
--- a/md2html/md2html.c
+++ b/md2html/md2html.c
@@ -440,8 +440,13 @@ static const option cmdline_options[] = {
     { "help",                       'h', 'h', OPTION_ARG_NONE },
     { "fverbatim-entities",          0,  'E', OPTION_ARG_NONE },
     { "fpermissive-atx-headers",     0,  'A', OPTION_ARG_NONE },
+    { "fpermissive-url-autolinks",   0,  'U', OPTION_ARG_NONE },
+    { "fpermissive-email-autolinks", 0,  '@', OPTION_ARG_NONE },
+    { "fpermissive-autolinks",       0,  'V', OPTION_ARG_NONE },
     { "fno-indented-code",           0,  'I', OPTION_ARG_NONE },
-    { "fno-html-blocks",             0,  'H', OPTION_ARG_NONE },
+    { "fno-html-blocks",             0,  'F', OPTION_ARG_NONE },
+    { "fno-html-spans",              0,  'G', OPTION_ARG_NONE },
+    { "fno-html",                    0,  'H', OPTION_ARG_NONE },
     { "fcollapse-whitespace",        0,  'W', OPTION_ARG_NONE },
     { 0 }
 };
@@ -454,17 +459,31 @@ usage(void)
         "Convert input FILE (or standard input) in Markdown format to HTML.\n"
         "\n"
         "General options:\n"
-        "  -o  --output=FILE        output file (default is standard output)\n"
-        "  -f, --full-html          generate full HTML document, including header\n"
-        "  -s, --stat               measure time of input parsing\n"
-        "  -h, --help               display this help and exit\n"
+        "  -o  --output=FILE    Output file (default is standard output)\n"
+        "  -f, --full-html      Generate full HTML document, including header\n"
+        "  -s, --stat           Measure time of input parsing\n"
+        "  -h, --help           Display this help and exit\n"
         "\n"
-        "Markdown dialect options:\n"
-        "      --fcollapse-whitespace       collapse non-trivial whitespace\n"
-        "      --fverbatim-entities         do not translate entities\n"
-        "      --fpermissive-atx-headers    allow ATX headers without delimiting space\n"
-        "      --fno-indented-code          disable indented code blocks\n"
-        "      --fno-html-blocks            disable raw HTML blocks\n"
+        "Markdown extension options:\n"
+        "      --fcollapse-whitespace\n"
+        "                       Collapse non-trivial whitespace\n"
+        "      --fverbatim-entities\n"
+        "                       Do not translate entities\n"
+        "      --fpermissive-atx-headers\n"
+        "                       Allow ATX headers without delimiting space\n"
+        "      --fpermissive-url-autolinks\n"
+        "                       Allow URL autolinks without '<', '>'\n"
+        "      --fpermissive-email-autolinks  \n"
+        "                       Allow e-mail autolinks without '<', '>' and 'mailto:'\n"
+        "      --fpermissive-autolinks\n"
+        "                       Same as --fpermissive-url-autolinks --fpermissive-email-autolinks\n"
+        "      --fno-indented-code\n"
+        "                       Disable indented code blocks\n"
+        "      --fno-html-blocks\n"
+        "                       Disable raw HTML blocks\n"
+        "      --fno-html-spans\n"
+        "                       Disable raw HTML spans\n"
+        "      --fno-html       Same as --fno-html-blocks --fno-html-spans\n"
     );
 }
 
@@ -492,8 +511,13 @@ cmdline_callback(int opt, char const* value, void* data)
         case 'E':   want_verbatim_entities = 1; break;
         case 'A':   renderer_flags |= MD_FLAG_PERMISSIVEATXHEADERS; break;
         case 'I':   renderer_flags |= MD_FLAG_NOINDENTEDCODEBLOCKS; break;
-        case 'H':   renderer_flags |= MD_FLAG_NOHTMLBLOCKS; break;
+        case 'F':   renderer_flags |= MD_FLAG_NOHTMLBLOCKS; break;
+        case 'G':   renderer_flags |= MD_FLAG_NOHTMLSPANS; break;
+        case 'H':   renderer_flags |= MD_FLAG_NOHTML; break;
         case 'W':   renderer_flags |= MD_FLAG_COLLAPSEWHITESPACE; break;
+        case 'U':   renderer_flags |= MD_FLAG_PERMISSIVEURLAUTOLINKS; break;
+        case '@':   renderer_flags |= MD_FLAG_PERMISSIVEEMAILAUTOLINKS; break;
+        case 'V':   renderer_flags |= MD_FLAG_PERMISSIVEURLAUTOLINKS | MD_FLAG_PERMISSIVEEMAILAUTOLINKS; break;
 
         default:
             fprintf(stderr, "Illegal option: %s\n", value);
diff --git a/md4c/md4c.c b/md4c/md4c.c
index a56745a..02fb991 100644
--- a/md4c/md4c.c
+++ b/md4c/md4c.c
@@ -91,6 +91,10 @@ struct MD_CTX_tag {
     MD_RENDERER r;
     void* userdata;
 
+    /* Helper temporary growing buffer. */
+    CHAR* buffer;
+    unsigned alloc_buffer;
+
     /* Stack of inline/span markers.
      * This is only used for parsing a single block contents but by storing it
      * here we may reuse the stack for subsequent blocks; i.e. we have fewer
@@ -253,6 +257,12 @@ md_str_case_eq(const CHAR* s1, const CHAR* s2, SZ n)
     return 0;
 }
 
+static inline int
+md_str_eq(const CHAR* s1, const CHAR* s2, SZ n)
+{
+    return memcmp(s1, s2, n * sizeof(CHAR));
+}
+
 static int
 md_text_with_null_replacement(MD_CTX* ctx, MD_TEXTTYPE type, const CHAR* str, SZ size)
 {
@@ -290,6 +300,26 @@ md_text_with_null_replacement(MD_CTX* ctx, MD_TEXTTYPE type, const CHAR* str, SZ
             goto abort;                                                 \
     } while(0)
 
+
+#define MD_TEMP_BUFFER(sz)                                              \
+    do {                                                                \
+        if(sz > ctx->alloc_buffer) {                                    \
+            CHAR* new_buffer;                                           \
+            SZ new_size = ((sz) + (sz) / 2 + 128) & ~127;               \
+                                                                        \
+            new_buffer = realloc(ctx->buffer, new_size);                \
+            if(new_buffer == NULL) {                                    \
+                MD_LOG("realloc() failed.");                            \
+                ret = -1;                                               \
+                goto abort;                                             \
+            }                                                           \
+                                                                        \
+            ctx->buffer = new_buffer;                                   \
+            ctx->alloc_buffer = new_size;                               \
+        }                                                               \
+    } while(0)
+
+
 #define MD_ENTER_BLOCK(type, arg)                                       \
     do {                                                                \
         ret = ctx->r.enter_block((type), (arg), ctx->userdata);         \
@@ -768,13 +798,15 @@ md_is_autolink(MD_CTX* ctx, OFF beg, OFF end)
 /* The mark structure.
  *
  * '\\': Maybe escape sequence.
+ * '\0': NULL char.
  *  '*': Maybe (strong) emphasis start/end.
  *  '`': Maybe code span start/end.
  *  '&': Maybe start of entity.
  *  ';': Maybe end of entity.
  *  '<': Maybe start of raw HTML or autolink.
  *  '>': Maybe end of raw HTML or autolink.
- *  '0': NULL char.
+ *  ':': Maybe permissive URL auto-link (needs MD_FLAG_PERMISSIVEURLAUTOLINKS)
+ *  '@': Maybe permissive e-mail auto-link (needs MD_FLAG_PERMISSIVEEMAILAUTOLINKS)
  *
  * Note that not all instances of these chars in the text imply creation of the
  * structure. Only those which have (or may have, after we see more context)
@@ -1161,6 +1193,54 @@ md_collect_marks(MD_CTX* ctx, const MD_LINE* lines, int n_lines)
                 }
             }
 
+            /* A potential permissive URL autolink. */
+            if((ctx->r.flags & MD_FLAG_PERMISSIVEURLAUTOLINKS)  &&  ch == _T(':')) {
+                static struct {
+                    const CHAR* scheme;
+                    SZ scheme_size;
+                    const CHAR* suffix;
+                    SZ suffix_size;
+                } scheme_map[] = {
+                    /* In the order from the most frequently used, arguably. */
+                    { _T("http"), 4,    _T("//"), 2 },
+                    { _T("https"), 5,   _T("//"), 2 },
+                    { _T("mailto"), 6,  NULL, 0 },
+                    { _T("ftp"), 3,     _T("//"), 2 },
+                };
+                int scheme_index;
+
+                for(scheme_index = 0; scheme_index < SIZEOF_ARRAY(scheme_map); scheme_index++) {
+                    const CHAR* scheme = scheme_map[scheme_index].scheme;
+                    const SZ scheme_size = scheme_map[scheme_index].scheme_size;
+                    const CHAR* suffix = scheme_map[scheme_index].suffix;
+                    const SZ suffix_size = scheme_map[scheme_index].suffix_size;
+
+                    if(line->beg + scheme_size <= off  &&  md_str_eq(STR(off-scheme_size), scheme, scheme_size) == 0  &&
+                        (line->beg + scheme_size == off || ISWHITESPACE(off-scheme_size-1))  &&
+                        off + 1 + suffix_size < line->end  &&  md_str_eq(STR(off+1), suffix, suffix_size) == 0)
+                    {
+                        PUSH_MARK(ch, off-scheme_size, off+1+suffix_size, MD_MARK_POTENTIAL_OPENER);
+                        /* Push a dummy as a reserve for a closer. */
+                        PUSH_MARK('D', off, off, 0);
+                        off += 1 + suffix_size;
+                        continue;
+                    }
+                }
+            }
+
+            /* A potential permissive e-mail autolink. */
+            if((ctx->r.flags & MD_FLAG_PERMISSIVEEMAILAUTOLINKS)  &&  ch == _T('@')) {
+                if(line->beg + 1 <= off  &&  ISALNUM(off-1)  &&
+                    off + 3 < line->end  &&  ISALNUM(off+1))
+                {
+                    PUSH_MARK(ch, off, off+1, MD_MARK_POTENTIAL_OPENER);
+                    /* Push a dummy as a reserve for a closer. */
+                    PUSH_MARK('D', off, off, 0);
+                    off++;
+                    continue;
+                }
+            }
+
             /* NULL character. */
             if(ch == _T('\0')) {
                 PUSH_MARK(ch, off, off+1, MD_MARK_RESOLVED);
@@ -1412,10 +1492,107 @@ md_analyze_underscore(MD_CTX* ctx, int mark_index)
     md_analyze_simple_pairing_mark(ctx, &UNDERSCORE_OPENERS, mark_index, 1);
 }
 
+static void
+md_analyze_permissive_url_autolink(MD_CTX* ctx, int mark_index)
+{
+    MD_MARK* opener = &ctx->marks[mark_index];
+    int closer_index;
+    MD_MARK* closer;
+    OFF off = opener->end;
+
+    if(off < ctx->size && ISALNUM(off))
+        off++;
+    else
+        return;
+
+    while(1) {
+        while(off < ctx->size && (ISALNUM(off) || CH(off) == _T('/')))
+            off++;
+
+        /* We need to be relatively careful to not include too much into the URL.
+         * Consider e.g. a dot or question mark:
+         *   "Go to http://example.com." versus "http://example.com.uk"
+         *   "Do you know http://zombo.com?" versus "http://example.com/?page=2"
+         * Therefore we include some named punctuation characters only if they
+         * are immediately followed by alnum char.
+         */
+        if(off + 1 < ctx->size && ISANYOF(off, _T("@.?=&%+-_#")) && ISALNUM(off+1))
+            off += 2;
+        else
+            break;
+    }
+
+    /* Ok. Lets call it auto-link. Adapt opener and create closer to zero
+     * length so all the contents becomes the link text. */
+    closer_index = md_split_mark(ctx, mark_index, 0);
+    closer = &ctx->marks[closer_index];
+
+    opener->end = opener->beg;
+    closer->beg = off;
+    closer->end = off;
+    md_resolve_range(ctx, NULL, mark_index, closer_index);
+}
+
+static void
+md_analyze_permissive_email_autolink(MD_CTX* ctx, int mark_index)
+{
+    MD_MARK* opener = &ctx->marks[mark_index];
+    int closer_index;
+    MD_MARK* closer;
+    OFF beg = opener->beg;
+    OFF end = opener->end;
+    int right_dot_count = 0;
+
+    MD_ASSERT(CH(beg) == _T('@'));
+
+    /* Accept any alphanumeric sequences delimited with dot before the '@'.
+     * There must be a whitespace or start of line before it. */
+    while(1) {
+        while(beg > 0  &&  ISALNUM(beg-1))
+            beg--;
+
+        if(beg > 1 && CH(beg-1) == _T('.') && ISALNUM(beg-2))
+            beg -= 2;
+        else if(beg == 0 || ISWHITESPACE(beg-1) || ISNEWLINE(beg-1))
+            break;
+        else
+            return;
+    }
+
+    /* Accept any alphanumeric sequences delimited with dot after the '@'. */
+    while(1) {
+        while(end + 1 < ctx->size  &&  ISALNUM(end))
+            end++;
+
+        if(end + 1 < ctx->size && CH(end) == _T('.') && ISALNUM(end+1)) {
+            right_dot_count++;
+            end += 2;
+        } else if(right_dot_count > 0) {
+            /* Although "user@machine" is technically correct e-mail address,
+             * we request at least one dot, as in e.g. "user@machine.com" to
+             * prevent some false positives with this very loose format. */
+            break;
+        } else {
+            return;
+        }
+    }
+
+    /* Ok. Lets call it auto-link. Adapt opener and create closer to zero
+     * length so all the contents becomes the link text. */
+    closer_index = md_split_mark(ctx, mark_index, 0);
+    closer = &ctx->marks[closer_index];
+
+    opener->beg = beg;
+    opener->end = beg;
+    closer->beg = end;
+    closer->end = end;
+    md_resolve_range(ctx, NULL, mark_index, closer_index);
+}
+
 /* Table of precedence of various span types. */
 static const CHAR* md_precedence_table[] = {
-    _T("&`<>"),     /* Code spans; autolinks; raw HTML. */
-    _T("*_")        /* Emphasis and string emphasis. */
+    _T("&`<>"),     /* Entities; code spans; autolinks; raw HTML. */
+    _T("*_@:")      /* Emphasis and string emphasis; permissive autolinks. */
 };
 
 static void
@@ -1452,6 +1629,8 @@ md_analyze_marks(MD_CTX* ctx, const MD_LINE* lines, int n_lines, int precedence_
             case '&':   md_analyze_entity(ctx, i); break;
             case '*':   md_analyze_asterisk(ctx, i); break;
             case '_':   md_analyze_underscore(ctx, i); break;
+            case ':':   md_analyze_permissive_url_autolink(ctx, i); break;
+            case '@':   md_analyze_permissive_email_autolink(ctx, i); break;
         }
 
         i++;
@@ -1563,24 +1742,42 @@ md_process_inlines(MD_CTX* ctx, const MD_LINE* lines, int n_lines)
                     }
                     break;
 
-                case '<':       /* Autolink or raw HTML. */
-                    if(mark->flags & MD_MARK_AUTOLINK) {
-                        det.a.href = STR(mark->end);
-                        det.a.href_size = ctx->marks[mark->next].beg - mark->end;
-                        MD_ENTER_SPAN(MD_SPAN_A, (void*) &det);
-                    } else {
-                        text_type = MD_TEXT_HTML;
+                case '<':
+                case '>':       /* Autolink or raw HTML. */
+                    if(!(mark->flags & MD_MARK_AUTOLINK)) {
+                        if(mark->flags & MD_MARK_OPENER)
+                            text_type = MD_TEXT_HTML;
+                        else
+                            text_type = MD_TEXT_NORMAL;
+                        break;
                     }
-                    break;
+                    /* Pass through, if auto-link. */
 
-                case '>':
-                    if(mark->flags & MD_MARK_AUTOLINK)
+                case '@':       /* Permissive e-mail autolink. */
+                case ':':       /* Permissive URL autolink. */
+                    if(mark->flags & MD_MARK_OPENER) {
+                        if(mark->ch == '@') {
+                            SZ sz = 7 + ctx->marks[mark->next].beg - mark->end;
+
+                            MD_TEMP_BUFFER(sz);
+                            memcpy(ctx->buffer, _T("mailto:"), 7 * sizeof(CHAR));
+                            memcpy(ctx->buffer + 7, STR(mark->end), (sz-7) * sizeof(CHAR));
+
+                            det.a.href_size = sz;
+                            det.a.href = ctx->buffer;
+                        } else {
+                            det.a.href_size = ctx->marks[mark->next].beg - mark->end;
+                            det.a.href = STR(mark->end);
+                        }
+                        det.a.title = NULL;
+                        det.a.title_size = 0;
+                        MD_ENTER_SPAN(MD_SPAN_A, (void*) &det);
+                    } else {
                         /* The detail already has to be initialized: There cannot
-                         * be any resolved mark between the autlink opener and
+                         * be any resolved mark between the autolink opener and
                          * closer. */
                         MD_LEAVE_SPAN(MD_SPAN_A, (void*) &det);
-                    else
-                        text_type = MD_TEXT_NORMAL;
+                    }
                     break;
 
                 case '&':       /* Entity. */
@@ -1930,7 +2127,7 @@ md_is_html_block_start_condition(MD_CTX* ctx, OFF beg)
 
         /* Check for type 5: <![CDATA[ */
         if(off + 8 < ctx->size) {
-            if(memcmp(STR(off), _T("![CDATA["), 8 * sizeof(CHAR)) == 0)
+            if(md_str_eq(STR(off), _T("![CDATA["), 8 * sizeof(CHAR)) == 0)
                 return 5;
         }
     }
@@ -2500,6 +2697,7 @@ md_parse(const MD_CHAR* text, MD_SIZE size, const MD_RENDERER* renderer, void* u
 
     /* Clean-up. */
     free(ctx.marks);
+    free(ctx.buffer);
 
     return ret;
 }
diff --git a/md4c/md4c.h b/md4c/md4c.h
index 23de4be..ec8ad3f 100644
--- a/md4c/md4c.h
+++ b/md4c/md4c.h
@@ -177,12 +177,15 @@ struct MD_BLOCK_CODE_DETAIL_tag {
  * By default (when MD_RENDERER::flags == 0), we follow CommonMark specification.
  * The following flags may allow some extensions or deviations from it.
  */
-#define MD_FLAG_COLLAPSEWHITESPACE      0x0001  /* In MD_TEXT_NORMAL, collapse non-trivial whitespace into single ' ' */
-#define MD_FLAG_PERMISSIVEATXHEADERS    0x0002  /* Do not require space in ATX headers ( ###header ) */
-#define MD_FLAG_NOINDENTEDCODEBLOCKS    0x0004  /* Disable indented code blocks. (Only fenced code works.) */
-#define MD_FLAG_NOHTMLBLOCKS            0x0010  /* Disable raw HTML blocks. */
-#define MD_FLAG_NOHTMLSPANS             0x0020  /* Disable raw HTML (inline). */
-#define MD_FLAG_NOHTML                  (MD_FLAG_NOHTMLBLOCKS | MD_FLAG_NOHTMLSPANS)
+#define MD_FLAG_COLLAPSEWHITESPACE          0x0001  /* In MD_TEXT_NORMAL, collapse non-trivial whitespace into single ' ' */
+#define MD_FLAG_PERMISSIVEATXHEADERS        0x0002  /* Do not require space in ATX headers ( ###header ) */
+#define MD_FLAG_PERMISSIVEURLAUTOLINKS      0x0004  /* Recognize URLs as autolinks even without '<', '>' */
+#define MD_FLAG_PERMISSIVEEMAILAUTOLINKS    0x0008  /* Recognize e-mails as autolinks even without '<', '>' and 'mailto:' */
+#define MD_FLAG_PERMISSIVEAUTOLINKS         (MD_FLAG_PERMISSIVEWWWAUTOLINKS | MD_FLAG_PERMISSIVEEMAILAUTOLINKS)
+#define MD_FLAG_NOINDENTEDCODEBLOCKS        0x0010  /* Disable indented code blocks. (Only fenced code works.) */
+#define MD_FLAG_NOHTMLBLOCKS                0x0020  /* Disable raw HTML blocks. */
+#define MD_FLAG_NOHTMLSPANS                 0x0040  /* Disable raw HTML (inline). */
+#define MD_FLAG_NOHTML                      (MD_FLAG_NOHTMLBLOCKS | MD_FLAG_NOHTMLSPANS)
 
 /* Caller-provided callbacks.
  *
diff --git a/scripts/run-tests.sh b/scripts/run-tests.sh
index 08195a3..1d26503 100755
--- a/scripts/run-tests.sh
+++ b/scripts/run-tests.sh
@@ -2,7 +2,7 @@
 #
 # Run this script from build directory.
 
-set -e
+#set -e
 
 SELF_DIR=`dirname $0`
 PROJECT_DIR="$SELF_DIR/.."
@@ -24,4 +24,9 @@ elif which python 2>/dev/null; then
     fi
 fi
 
-$PYTHON "$TEST_DIR/spec_tests.py" -s "$TEST_DIR/spec.txt" -p "$PROGRAM" "$@"
+# Test CommonMark specification compliance (with default options):
+$PYTHON "$TEST_DIR/spec_tests.py" -s "$TEST_DIR/spec.txt" -p "$PROGRAM"
+
+# Test various extensions and deviations from the specifications:
+$PYTHON "$TEST_DIR/spec_tests.py" -s "$TEST_DIR/permissive-email-autolinks.txt" -p "$PROGRAM --fpermissive-email-autolinks"
+$PYTHON "$TEST_DIR/spec_tests.py" -s "$TEST_DIR/permissive-url-autolinks.txt" -p "$PROGRAM --fpermissive-url-autolinks"
diff --git a/test/permissive-email-autolinks.txt b/test/permissive-email-autolinks.txt
new file mode 100644
index 0000000..7d7f572
--- /dev/null
+++ b/test/permissive-email-autolinks.txt
@@ -0,0 +1,22 @@
+
+# Permissive E-mail Autolinks
+
+With the flag `MD_FLAG_PERMISSIVEEMAILAUTOLINKS`, MD4C enables more permissive recognition
+of e-mail addresses and transforms them to autolinks, even if they do not exactly follow
+the syntax of autolink as specified in CommonMark specification.
+
+This is standard CommonMark e-mail autolink:
+
+```````````````````````````````` example
+E-mail: <mailto:john.doe@gmail.com>
+.
+<p>E-mail: <a href="mailto:john.doe@gmail.com">mailto:john.doe@gmail.com</a></p>
+````````````````````````````````
+
+With the permissive autolinks enabled, this is sufficient:
+
+```````````````````````````````` example
+E-mail: john.doe@gmail.com
+.
+<p>E-mail: <a href="mailto:john.doe@gmail.com">john.doe@gmail.com</a></p>
+````````````````````````````````
diff --git a/test/permissive-url-autolinks.txt b/test/permissive-url-autolinks.txt
new file mode 100644
index 0000000..4b42f95
--- /dev/null
+++ b/test/permissive-url-autolinks.txt
@@ -0,0 +1,72 @@
+
+# Permissive URL Autolinks
+
+With the flag `MD_FLAG_PERMISSIVEURLAUTOLINKS`, MD4C enables more permissive recognition
+of URLs and transform them to autolinks, even if they do not exactly follow the syntax
+of autolink as specified in CommonMark specification.
+
+This is standard CommonMark autolink:
+
+```````````````````````````````` example
+Homepage: <https://github.com/mity/md4c>
+.
+<p>Homepage: <a href="https://github.com/mity/md4c">https://github.com/mity/md4c</a></p>
+````````````````````````````````
+
+With the permissive autolinks enabled, this is sufficient:
+
+```````````````````````````````` example
+Homepage: https://github.com/mity/md4c
+.
+<p>Homepage: <a href="https://github.com/mity/md4c">https://github.com/mity/md4c</a></p>
+````````````````````````````````
+
+But this permissive autolink feature can work only for very widely used URL
+schemes, in alphabetical order `ftp:`, `http:`, `https:` and `mailto:`,
+so this is not an permissive autolink.
+
+That's why this is not a permissive autolink:
+
+```````````````````````````````` example
+ssh://root@example.com
+.
+<p>ssh://root@example.com</p>
+````````````````````````````````
+
+As certain characters (`.`, `?`, `=`, `&`, `%`, `+`, `-`, `_`, `#`, `@`) can collide
+with a normal text flow, they are not recognized as part of the URL unless another
+alphanumeric character immediately follows.
+
+Therefore the question mark in this question is not part of the autolink's
+contents:
+
+```````````````````````````````` example
+Have you ever visited http://www.zombo.com?
+.
+<p>Have you ever visited <a href="http://www.zombo.com">http://www.zombo.com</a>?</p>
+````````````````````````````````
+
+But in contrast, in this example it is:
+
+```````````````````````````````` example
+http://www.bing.com/search?q=md4c
+.
+<p><a href="http://www.bing.com/search?q=md4c">http://www.bing.com/search?q=md4c</a></p>
+````````````````````````````````
+
+Please note that e-mail addresses without the `mailto:` scheme are not permissive URL
+autolinks, but MD4C implements another extension `MD_FLAG_PERMISSIVEEMAILAUTOLINKS`
+which can be used to enable that too.
+
+```````````````````````````````` example
+mailto:john.doe@example.com
+.
+<p><a href="mailto:john.doe@example.com">mailto:john.doe@example.com</a></p>
+````````````````````````````````
+
+```````````````````````````````` example
+john.doe@example.com
+.
+<p>john.doe@example.com</p>
+````````````````````````````````
+