/lib/markdown2.py

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#!/usr/bin/env python
# Copyright (c) 2007-2008 ActiveState Corp.
# License: MIT (http://www.opensource.org/licenses/mit-license.php)

r"""A fast and complete Python implementation of Markdown.

[from http://daringfireball.net/projects/markdown/]
> Markdown is a text-to-HTML filter; it translates an easy-to-read /
> easy-to-write structured text format into HTML.  Markdown's text
> format is most similar to that of plain text email, and supports
> features such as headers, *emphasis*, code blocks, blockquotes, and
> links.
>
> Markdown's syntax is designed not as a generic markup language, but
> specifically to serve as a front-end to (X)HTML. You can use span-level
> HTML tags anywhere in a Markdown document, and you can use block level
> HTML tags (like <div> and <table> as well).

Module usage:

    >>> import markdown2
    >>> markdown2.markdown("*boo!*")  # or use `html = markdown_path(PATH)`
    u'<p><em>boo!</em></p>\n'

    >>> markdowner = Markdown()
    >>> markdowner.convert("*boo!*")
    u'<p><em>boo!</em></p>\n'
    >>> markdowner.convert("**boom!**")
    u'<p><strong>boom!</strong></p>\n'

This implementation of Markdown implements the full "core" syntax plus a
number of extras (e.g., code syntax coloring, footnotes) as described on
<http://code.google.com/p/python-markdown2/wiki/Extras>.
"""

cmdln_desc = """A fast and complete Python implementation of Markdown, a
text-to-HTML conversion tool for web writers.

Supported extras (see -x|--extras option below):
* code-friendly: Disable _ and __ for em and strong.
* code-color: Pygments-based syntax coloring of <code> sections.
* cuddled-lists: Allow lists to be cuddled to the preceding paragraph.
* footnotes: Support footnotes as in use on daringfireball.net and
  implemented in other Markdown processors (tho not in Markdown.pl v1.0.1).
* header-ids: Adds "id" attributes to headers. The id value is a slug of
  the header text.
* html-classes: Takes a dict mapping html tag names (lowercase) to a
  string to use for a "class" tag attribute. Currently only supports
  "pre" and "code" tags. Add an issue if you require this for other tags.
* markdown-in-html: Allow the use of `markdown="1"` in a block HTML tag to
  have markdown processing be done on its contents. Similar to
  <http://michelf.com/projects/php-markdown/extra/#markdown-attr> but with
  some limitations.
* pyshell: Treats unindented Python interactive shell sessions as <code>
  blocks.
* link-patterns: Auto-link given regex patterns in text (e.g. bug number
  references, revision number references).
* smarty-pants: Replaces ' and " with curly quotation marks or curly 
  apostrophes.  Replaces --, ---, ..., and . . . with en dashes, em dashes, 
  and ellipses.
* toc: The returned HTML string gets a new "toc_html" attribute which is
  a Table of Contents for the document. (experimental)
* xml: Passes one-liner processing instructions and namespaced XML tags.
"""

# Dev Notes:
# - There is already a Python markdown processor
#   (http://www.freewisdom.org/projects/python-markdown/).
# - Python's regex syntax doesn't have '\z', so I'm using '\Z'. I'm
#   not yet sure if there implications with this. Compare 'pydoc sre'
#   and 'perldoc perlre'.

__version_info__ = (1, 0, 1, 18) # first three nums match Markdown.pl
__version__ = '1.0.1.18'
__author__ = "Trent Mick"

import os
import sys
from pprint import pprint
import re
import logging
try:
    from hashlib import md5
except ImportError:
    from md5 import md5
import optparse
from random import random, randint
import codecs
from urllib import quote



#---- Python version compat

if sys.version_info[:2] < (2,4):
    from sets import Set as set
    def reversed(sequence):
        for i in sequence[::-1]:
            yield i
    def _unicode_decode(s, encoding, errors='xmlcharrefreplace'):
        return unicode(s, encoding, errors)
else:
    def _unicode_decode(s, encoding, errors='strict'):
        return s.decode(encoding, errors)


#---- globals

DEBUG = False
log = logging.getLogger("markdown")

DEFAULT_TAB_WIDTH = 4


try:
    import uuid
except ImportError:
    SECRET_SALT = str(randint(0, 1000000))
else:
    SECRET_SALT = str(uuid.uuid4())
def _hash_ascii(s):
    #return md5(s).hexdigest()   # Markdown.pl effectively does this.
    return 'md5-' + md5(SECRET_SALT + s).hexdigest()
def _hash_text(s):
    return 'md5-' + md5(SECRET_SALT + s.encode("utf-8")).hexdigest()

# Table of hash values for escaped characters:
g_escape_table = dict([(ch, _hash_ascii(ch))
                       for ch in '\\`*_{}[]()>#+-.!'])



#---- exceptions

class MarkdownError(Exception):
    pass



#---- public api

def markdown_path(path, encoding="utf-8",
                  html4tags=False, tab_width=DEFAULT_TAB_WIDTH,
                  safe_mode=None, extras=None, link_patterns=None,
                  use_file_vars=False):
    fp = codecs.open(path, 'r', encoding)
    text = fp.read()
    fp.close()
    return Markdown(html4tags=html4tags, tab_width=tab_width,
                    safe_mode=safe_mode, extras=extras,
                    link_patterns=link_patterns,
                    use_file_vars=use_file_vars).convert(text)

def markdown(text, html4tags=False, tab_width=DEFAULT_TAB_WIDTH,
             safe_mode=None, extras=None, link_patterns=None,
             use_file_vars=False):
    return Markdown(html4tags=html4tags, tab_width=tab_width,
                    safe_mode=safe_mode, extras=extras,
                    link_patterns=link_patterns,
                    use_file_vars=use_file_vars).convert(text)

class Markdown(object):
    # The dict of "extras" to enable in processing -- a mapping of
    # extra name to argument for the extra. Most extras do not have an
    # argument, in which case the value is None.
    #
    # This can be set via (a) subclassing and (b) the constructor
    # "extras" argument.
    extras = None

    urls = None
    titles = None
    html_blocks = None
    html_spans = None
    html_removed_text = "[HTML_REMOVED]"  # for compat with markdown.py

    # Used to track when we're inside an ordered or unordered list
    # (see _ProcessListItems() for details):
    list_level = 0

    _ws_only_line_re = re.compile(r"^[ \t]+$", re.M)

    def __init__(self, html4tags=False, tab_width=4, safe_mode=None,
                 extras=None, link_patterns=None, use_file_vars=False):
        if html4tags:
            self.empty_element_suffix = ">"
        else:
            self.empty_element_suffix = " />"
        self.tab_width = tab_width

        # For compatibility with earlier markdown2.py and with
        # markdown.py's safe_mode being a boolean, 
        #   safe_mode == True -> "replace"
        if safe_mode is True:
            self.safe_mode = "replace"
        else:
            self.safe_mode = safe_mode

        # Massaging and building the "extras" info.
        if self.extras is None:
            self.extras = {}
        elif not isinstance(self.extras, dict):
            self.extras = dict([(e, None) for e in self.extras])
        if extras:
            if not isinstance(extras, dict):
                extras = dict([(e, None) for e in extras])
            self.extras.update(extras)
        assert isinstance(self.extras, dict)
        if "toc" in self.extras and not "header-ids" in self.extras:
            self.extras["header-ids"] = None   # "toc" implies "header-ids"
        self._instance_extras = self.extras.copy()
        
        self.link_patterns = link_patterns
        self.use_file_vars = use_file_vars
        self._outdent_re = re.compile(r'^(\t|[ ]{1,%d})' % tab_width, re.M)

        self._escape_table = g_escape_table.copy()
        if "smarty-pants" in self.extras:
            self._escape_table['"'] = _hash_ascii('"')
            self._escape_table["'"] = _hash_ascii("'")

    def reset(self):
        self.urls = {}
        self.titles = {}
        self.html_blocks = {}
        self.html_spans = {}
        self.list_level = 0
        self.extras = self._instance_extras.copy()
        if "footnotes" in self.extras:
            self.footnotes = {}
            self.footnote_ids = []
        if "header-ids" in self.extras:
            self._count_from_header_id = {} # no `defaultdict` in Python 2.4

    def convert(self, text):
        """Convert the given text."""
        # Main function. The order in which other subs are called here is
        # essential. Link and image substitutions need to happen before
        # _EscapeSpecialChars(), so that any *'s or _'s in the <a>
        # and <img> tags get encoded.

        # Clear the global hashes. If we don't clear these, you get conflicts
        # from other articles when generating a page which contains more than
        # one article (e.g. an index page that shows the N most recent
        # articles):
        self.reset()

        if not isinstance(text, unicode):
            #TODO: perhaps shouldn't presume UTF-8 for string input?
            text = unicode(text, 'utf-8')

        if self.use_file_vars:
            # Look for emacs-style file variable hints.
            emacs_vars = self._get_emacs_vars(text)
            if "markdown-extras" in emacs_vars:
                splitter = re.compile("[ ,]+")
                for e in splitter.split(emacs_vars["markdown-extras"]):
                    if '=' in e:
                        ename, earg = e.split('=', 1)
                        try:
                            earg = int(earg)
                        except ValueError:
                            pass
                    else:
                        ename, earg = e, None
                    self.extras[ename] = earg

        # Standardize line endings:
        text = re.sub("\r\n|\r", "\n", text)

        # Make sure $text ends with a couple of newlines:
        text += "\n\n"

        # Convert all tabs to spaces.
        text = self._detab(text)

        # Strip any lines consisting only of spaces and tabs.
        # This makes subsequent regexen easier to write, because we can
        # match consecutive blank lines with /\n+/ instead of something
        # contorted like /[ \t]*\n+/ .
        text = self._ws_only_line_re.sub("", text)

        if self.safe_mode:
            text = self._hash_html_spans(text)

        # Turn block-level HTML blocks into hash entries
        text = self._hash_html_blocks(text, raw=True)

        # Strip link definitions, store in hashes.
        if "footnotes" in self.extras:
            # Must do footnotes first because an unlucky footnote defn
            # looks like a link defn:
            #   [^4]: this "looks like a link defn"
            text = self._strip_footnote_definitions(text)
        text = self._strip_link_definitions(text)

        text = self._run_block_gamut(text)

        if "footnotes" in self.extras:
            text = self._add_footnotes(text)

        text = self.postprocess(text)

        text = self._unescape_special_chars(text)

        if self.safe_mode:
            text = self._unhash_html_spans(text)

        text += "\n"
        
        rv = UnicodeWithAttrs(text)
        if "toc" in self.extras:
            rv._toc = self._toc
        return rv

    def postprocess(self, text):
        """A hook for subclasses to do some postprocessing of the html, if
        desired. This is called before unescaping of special chars and
        unhashing of raw HTML spans.
        """
        return text

    _emacs_oneliner_vars_pat = re.compile(r"-\*-\s*([^\r\n]*?)\s*-\*-", re.UNICODE)
    # This regular expression is intended to match blocks like this:
    #    PREFIX Local Variables: SUFFIX
    #    PREFIX mode: Tcl SUFFIX
    #    PREFIX End: SUFFIX
    # Some notes:
    # - "[ \t]" is used instead of "\s" to specifically exclude newlines
    # - "(\r\n|\n|\r)" is used instead of "$" because the sre engine does
    #   not like anything other than Unix-style line terminators.
    _emacs_local_vars_pat = re.compile(r"""^
        (?P<prefix>(?:[^\r\n|\n|\r])*?)
        [\ \t]*Local\ Variables:[\ \t]*
        (?P<suffix>.*?)(?:\r\n|\n|\r)
        (?P<content>.*?\1End:)
        """, re.IGNORECASE | re.MULTILINE | re.DOTALL | re.VERBOSE)

    def _get_emacs_vars(self, text):
        """Return a dictionary of emacs-style local variables.

        Parsing is done loosely according to this spec (and according to
        some in-practice deviations from this):
        http://www.gnu.org/software/emacs/manual/html_node/emacs/Specifying-File-Variables.html#Specifying-File-Variables
        """
        emacs_vars = {}
        SIZE = pow(2, 13) # 8kB

        # Search near the start for a '-*-'-style one-liner of variables.
        head = text[:SIZE]
        if "-*-" in head:
            match = self._emacs_oneliner_vars_pat.search(head)
            if match:
                emacs_vars_str = match.group(1)
                assert '\n' not in emacs_vars_str
                emacs_var_strs = [s.strip() for s in emacs_vars_str.split(';')
                                  if s.strip()]
                if len(emacs_var_strs) == 1 and ':' not in emacs_var_strs[0]:
                    # While not in the spec, this form is allowed by emacs:
                    #   -*- Tcl -*-
                    # where the implied "variable" is "mode". This form
                    # is only allowed if there are no other variables.
                    emacs_vars["mode"] = emacs_var_strs[0].strip()
                else:
                    for emacs_var_str in emacs_var_strs:
                        try:
                            variable, value = emacs_var_str.strip().split(':', 1)
                        except ValueError:
                            log.debug("emacs variables error: malformed -*- "
                                      "line: %r", emacs_var_str)
                            continue
                        # Lowercase the variable name because Emacs allows "Mode"
                        # or "mode" or "MoDe", etc.
                        emacs_vars[variable.lower()] = value.strip()

        tail = text[-SIZE:]
        if "Local Variables" in tail:
            match = self._emacs_local_vars_pat.search(tail)
            if match:
                prefix = match.group("prefix")
                suffix = match.group("suffix")
                lines = match.group("content").splitlines(0)
                #print "prefix=%r, suffix=%r, content=%r, lines: %s"\
                #      % (prefix, suffix, match.group("content"), lines)

                # Validate the Local Variables block: proper prefix and suffix
                # usage.
                for i, line in enumerate(lines):
                    if not line.startswith(prefix):
                        log.debug("emacs variables error: line '%s' "
                                  "does not use proper prefix '%s'"
                                  % (line, prefix))
                        return {}
                    # Don't validate suffix on last line. Emacs doesn't care,
                    # neither should we.
                    if i != len(lines)-1 and not line.endswith(suffix):
                        log.debug("emacs variables error: line '%s' "
                                  "does not use proper suffix '%s'"
                                  % (line, suffix))
                        return {}

                # Parse out one emacs var per line.
                continued_for = None
                for line in lines[:-1]: # no var on the last line ("PREFIX End:")
                    if prefix: line = line[len(prefix):] # strip prefix
                    if suffix: line = line[:-len(suffix)] # strip suffix
                    line = line.strip()
                    if continued_for:
                        variable = continued_for
                        if line.endswith('\\'):
                            line = line[:-1].rstrip()
                        else:
                            continued_for = None
                        emacs_vars[variable] += ' ' + line
                    else:
                        try:
                            variable, value = line.split(':', 1)
                        except ValueError:
                            log.debug("local variables error: missing colon "
                                      "in local variables entry: '%s'" % line)
                            continue
                        # Do NOT lowercase the variable name, because Emacs only
                        # allows "mode" (and not "Mode", "MoDe", etc.) in this block.
                        value = value.strip()
                        if value.endswith('\\'):
                            value = value[:-1].rstrip()
                            continued_for = variable
                        else:
                            continued_for = None
                        emacs_vars[variable] = value

        # Unquote values.
        for var, val in emacs_vars.items():
            if len(val) > 1 and (val.startswith('"') and val.endswith('"')
               or val.startswith('"') and val.endswith('"')):
                emacs_vars[var] = val[1:-1]

        return emacs_vars

    # Cribbed from a post by Bart Lateur:
    # <http://www.nntp.perl.org/group/perl.macperl.anyperl/154>
    _detab_re = re.compile(r'(.*?)\t', re.M)
    def _detab_sub(self, match):
        g1 = match.group(1)
        return g1 + (' ' * (self.tab_width - len(g1) % self.tab_width))
    def _detab(self, text):
        r"""Remove (leading?) tabs from a file.

            >>> m = Markdown()
            >>> m._detab("\tfoo")
            '    foo'
            >>> m._detab("  \tfoo")
            '    foo'
            >>> m._detab("\t  foo")
            '      foo'
            >>> m._detab("  foo")
            '  foo'
            >>> m._detab("  foo\n\tbar\tblam")
            '  foo\n    bar blam'
        """
        if '\t' not in text:
            return text
        return self._detab_re.subn(self._detab_sub, text)[0]

    _block_tags_a = 'p|div|h[1-6]|blockquote|pre|table|dl|ol|ul|script|noscript|form|fieldset|iframe|math|ins|del'
    _strict_tag_block_re = re.compile(r"""
        (                       # save in \1
            ^                   # start of line  (with re.M)
            <(%s)               # start tag = \2
            \b                  # word break
            (.*\n)*?            # any number of lines, minimally matching
            </\2>               # the matching end tag
            [ \t]*              # trailing spaces/tabs
            (?=\n+|\Z)          # followed by a newline or end of document
        )
        """ % _block_tags_a,
        re.X | re.M)

    _block_tags_b = 'p|div|h[1-6]|blockquote|pre|table|dl|ol|ul|script|noscript|form|fieldset|iframe|math'
    _liberal_tag_block_re = re.compile(r"""
        (                       # save in \1
            ^                   # start of line  (with re.M)
            <(%s)               # start tag = \2
            \b                  # word break
            (.*\n)*?            # any number of lines, minimally matching
            .*</\2>             # the matching end tag
            [ \t]*              # trailing spaces/tabs
            (?=\n+|\Z)          # followed by a newline or end of document
        )
        """ % _block_tags_b,
        re.X | re.M)

    _html_markdown_attr_re = re.compile(
        r'''\s+markdown=("1"|'1')''')
    def _hash_html_block_sub(self, match, raw=False):
        html = match.group(1)
        if raw and self.safe_mode:
            html = self._sanitize_html(html)
        elif 'markdown-in-html' in self.extras and 'markdown=' in html:
            first_line = html.split('\n', 1)[0]
            m = self._html_markdown_attr_re.search(first_line)
            if m:
                lines = html.split('\n')
                middle = '\n'.join(lines[1:-1])
                last_line = lines[-1]
                first_line = first_line[:m.start()] + first_line[m.end():]
                f_key = _hash_text(first_line)
                self.html_blocks[f_key] = first_line
                l_key = _hash_text(last_line)
                self.html_blocks[l_key] = last_line
                return ''.join(["\n\n", f_key,
                    "\n\n", middle, "\n\n",
                    l_key, "\n\n"])
        key = _hash_text(html)
        self.html_blocks[key] = html
        return "\n\n" + key + "\n\n"

    def _hash_html_blocks(self, text, raw=False):
        """Hashify HTML blocks

        We only want to do this for block-level HTML tags, such as headers,
        lists, and tables. That's because we still want to wrap <p>s around
        "paragraphs" that are wrapped in non-block-level tags, such as anchors,
        phrase emphasis, and spans. The list of tags we're looking for is
        hard-coded.

        @param raw {boolean} indicates if these are raw HTML blocks in
            the original source. It makes a difference in "safe" mode.
        """
        if '<' not in text:
            return text

        # Pass `raw` value into our calls to self._hash_html_block_sub.
        hash_html_block_sub = _curry(self._hash_html_block_sub, raw=raw)

        # First, look for nested blocks, e.g.:
        #   <div>
        #       <div>
        #       tags for inner block must be indented.
        #       </div>
        #   </div>
        #
        # The outermost tags must start at the left margin for this to match, and
        # the inner nested divs must be indented.
        # We need to do this before the next, more liberal match, because the next
        # match will start at the first `<div>` and stop at the first `</div>`.
        text = self._strict_tag_block_re.sub(hash_html_block_sub, text)

        # Now match more liberally, simply from `\n<tag>` to `</tag>\n`
        text = self._liberal_tag_block_re.sub(hash_html_block_sub, text)

        # Special case just for <hr />. It was easier to make a special
        # case than to make the other regex more complicated.   
        if "<hr" in text:
            _hr_tag_re = _hr_tag_re_from_tab_width(self.tab_width)
            text = _hr_tag_re.sub(hash_html_block_sub, text)

        # Special case for standalone HTML comments:
        if "<!--" in text:
            start = 0
            while True:
                # Delimiters for next comment block.
                try:
                    start_idx = text.index("<!--", start)
                except ValueError, ex:
                    break
                try:
                    end_idx = text.index("-->", start_idx) + 3
                except ValueError, ex:
                    break

                # Start position for next comment block search.
                start = end_idx

                # Validate whitespace before comment.
                if start_idx:
                    # - Up to `tab_width - 1` spaces before start_idx.
                    for i in range(self.tab_width - 1):
                        if text[start_idx - 1] != ' ':
                            break
                        start_idx -= 1
                        if start_idx == 0:
                            break
                    # - Must be preceded by 2 newlines or hit the start of
                    #   the document.
                    if start_idx == 0:
                        pass
                    elif start_idx == 1 and text[0] == '\n':
                        start_idx = 0  # to match minute detail of Markdown.pl regex
                    elif text[start_idx-2:start_idx] == '\n\n':
                        pass
                    else:
                        break

                # Validate whitespace after comment.
                # - Any number of spaces and tabs.
                while end_idx < len(text):
                    if text[end_idx] not in ' \t':
                        break
                    end_idx += 1
                # - Must be following by 2 newlines or hit end of text.
                if text[end_idx:end_idx+2] not in ('', '\n', '\n\n'):
                    continue

                # Escape and hash (must match `_hash_html_block_sub`).
                html = text[start_idx:end_idx]
                if raw and self.safe_mode:
                    html = self._sanitize_html(html)
                key = _hash_text(html)
                self.html_blocks[key] = html
                text = text[:start_idx] + "\n\n" + key + "\n\n" + text[end_idx:]

        if "xml" in self.extras:
            # Treat XML processing instructions and namespaced one-liner
            # tags as if they were block HTML tags. E.g., if standalone
            # (i.e. are their own paragraph), the following do not get 
            # wrapped in a <p> tag:
            #    <?foo bar?>
            #
            #    <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="chapter_1.md"/>
            _xml_oneliner_re = _xml_oneliner_re_from_tab_width(self.tab_width)
            text = _xml_oneliner_re.sub(hash_html_block_sub, text)

        return text

    def _strip_link_definitions(self, text):
        # Strips link definitions from text, stores the URLs and titles in
        # hash references.
        less_than_tab = self.tab_width - 1
    
        # Link defs are in the form:
        #   [id]: url "optional title"
        _link_def_re = re.compile(r"""
            ^[ ]{0,%d}\[(.+)\]: # id = \1
              [ \t]*
              \n?               # maybe *one* newline
              [ \t]*
            <?(.+?)>?           # url = \2
              [ \t]*
            (?:
                \n?             # maybe one newline
                [ \t]*
                (?<=\s)         # lookbehind for whitespace
                ['"(]
                ([^\n]*)        # title = \3
                ['")]
                [ \t]*
            )?  # title is optional
            (?:\n+|\Z)
            """ % less_than_tab, re.X | re.M | re.U)
        return _link_def_re.sub(self._extract_link_def_sub, text)

    def _extract_link_def_sub(self, match):
        id, url, title = match.groups()
        key = id.lower()    # Link IDs are case-insensitive
        self.urls[key] = self._encode_amps_and_angles(url)
        if title:
            self.titles[key] = title.replace('"', '&quot;')
        return ""

    def _extract_footnote_def_sub(self, match):
        id, text = match.groups()
        text = _dedent(text, skip_first_line=not text.startswith('\n')).strip()
        normed_id = re.sub(r'\W', '-', id)
        # Ensure footnote text ends with a couple newlines (for some
        # block gamut matches).
        self.footnotes[normed_id] = text + "\n\n"
        return ""

    def _strip_footnote_definitions(self, text):
        """A footnote definition looks like this:

            [^note-id]: Text of the note.

                May include one or more indented paragraphs.

        Where,
        - The 'note-id' can be pretty much anything, though typically it
          is the number of the footnote.
        - The first paragraph may start on the next line, like so:
            
            [^note-id]:
                Text of the note.
        """
        less_than_tab = self.tab_width - 1
        footnote_def_re = re.compile(r'''
            ^[ ]{0,%d}\[\^(.+)\]:   # id = \1
            [ \t]*
            (                       # footnote text = \2
              # First line need not start with the spaces.
              (?:\s*.*\n+)
              (?:
                (?:[ ]{%d} | \t)  # Subsequent lines must be indented.
                .*\n+
              )*
            )
            # Lookahead for non-space at line-start, or end of doc.
            (?:(?=^[ ]{0,%d}\S)|\Z)
            ''' % (less_than_tab, self.tab_width, self.tab_width),
            re.X | re.M)
        return footnote_def_re.sub(self._extract_footnote_def_sub, text)


    _hr_res = [
        re.compile(r"^[ ]{0,2}([ ]?\*[ ]?){3,}[ \t]*$", re.M),
        re.compile(r"^[ ]{0,2}([ ]?\-[ ]?){3,}[ \t]*$", re.M),
        re.compile(r"^[ ]{0,2}([ ]?\_[ ]?){3,}[ \t]*$", re.M),
    ]

    def _run_block_gamut(self, text):
        # These are all the transformations that form block-level
        # tags like paragraphs, headers, and list items.

        text = self._do_headers(text)

        # Do Horizontal Rules:
        hr = "\n<hr"+self.empty_element_suffix+"\n"
        for hr_re in self._hr_res:
            text = hr_re.sub(hr, text)

        text = self._do_lists(text)

        if "pyshell" in self.extras:
            text = self._prepare_pyshell_blocks(text)

        text = self._do_code_blocks(text)

        text = self._do_block_quotes(text)

        # We already ran _HashHTMLBlocks() before, in Markdown(), but that
        # was to escape raw HTML in the original Markdown source. This time,
        # we're escaping the markup we've just created, so that we don't wrap
        # <p> tags around block-level tags.
        text = self._hash_html_blocks(text)

        text = self._form_paragraphs(text)

        return text

    def _pyshell_block_sub(self, match):
        lines = match.group(0).splitlines(0)
        _dedentlines(lines)
        indent = ' ' * self.tab_width
        s = ('\n' # separate from possible cuddled paragraph
             + indent + ('\n'+indent).join(lines)
             + '\n\n')
        return s
        
    def _prepare_pyshell_blocks(self, text):
        """Ensure that Python interactive shell sessions are put in
        code blocks -- even if not properly indented.
        """
        if ">>>" not in text:
            return text

        less_than_tab = self.tab_width - 1
        _pyshell_block_re = re.compile(r"""
            ^([ ]{0,%d})>>>[ ].*\n   # first line
            ^(\1.*\S+.*\n)*         # any number of subsequent lines
            ^\n                     # ends with a blank line
            """ % less_than_tab, re.M | re.X)

        return _pyshell_block_re.sub(self._pyshell_block_sub, text)

    def _run_span_gamut(self, text):
        # These are all the transformations that occur *within* block-level
        # tags like paragraphs, headers, and list items.
    
        text = self._do_code_spans(text)
    
        text = self._escape_special_chars(text)
    
        # Process anchor and image tags.
        text = self._do_links(text)
    
        # Make links out of things like `<http://example.com/>`
        # Must come after _do_links(), because you can use < and >
        # delimiters in inline links like [this](<url>).
        text = self._do_auto_links(text)

        if "link-patterns" in self.extras:
            text = self._do_link_patterns(text)
    
        text = self._encode_amps_and_angles(text)
    
        text = self._do_italics_and_bold(text)
    
        if "smarty-pants" in self.extras:
            text = self._do_smart_punctuation(text)
    
        # Do hard breaks:
        text = re.sub(r" {2,}\n", " <br%s\n" % self.empty_element_suffix, text)
    
        return text

    # "Sorta" because auto-links are identified as "tag" tokens.
    _sorta_html_tokenize_re = re.compile(r"""
        (
            # tag
            </?         
            (?:\w+)                                     # tag name
            (?:\s+(?:[\w-]+:)?[\w-]+=(?:".*?"|'.*?'))*  # attributes
            \s*/?>
            |
            # auto-link (e.g., <http://www.activestate.com/>)
            <\w+[^>]*>
            |
            <!--.*?-->      # comment
            |
            <\?.*?\?>       # processing instruction
        )
        """, re.X)
    
    def _escape_special_chars(self, text):
        # Python markdown note: the HTML tokenization here differs from
        # that in Markdown.pl, hence the behaviour for subtle cases can
        # differ (I believe the tokenizer here does a better job because
        # it isn't susceptible to unmatched '<' and '>' in HTML tags).
        # Note, however, that '>' is not allowed in an auto-link URL
        # here.
        escaped = []
        is_html_markup = False
        for token in self._sorta_html_tokenize_re.split(text):
            if is_html_markup:
                # Within tags/HTML-comments/auto-links, encode * and _
                # so they don't conflict with their use in Markdown for
                # italics and strong.  We're replacing each such
                # character with its corresponding MD5 checksum value;
                # this is likely overkill, but it should prevent us from
                # colliding with the escape values by accident.
                escaped.append(token.replace('*', self._escape_table['*'])
                                    .replace('_', self._escape_table['_']))
            else:
                escaped.append(self._encode_backslash_escapes(token))
            is_html_markup = not is_html_markup
        return ''.join(escaped)

    def _hash_html_spans(self, text):
        # Used for safe_mode.

        def _is_auto_link(s):
            if ':' in s and self._auto_link_re.match(s):
                return True
            elif '@' in s and self._auto_email_link_re.match(s):
                return True
            return False

        tokens = []
        is_html_markup = False
        for token in self._sorta_html_tokenize_re.split(text):
            if is_html_markup and not _is_auto_link(token):
                sanitized = self._sanitize_html(token)
                key = _hash_text(sanitized)
                self.html_spans[key] = sanitized
                tokens.append(key)
            else:
                tokens.append(token)
            is_html_markup = not is_html_markup
        return ''.join(tokens)

    def _unhash_html_spans(self, text):
        for key, sanitized in self.html_spans.items():
            text = text.replace(key, sanitized)
        return text

    def _sanitize_html(self, s):
        if self.safe_mode == "replace":
            return self.html_removed_text
        elif self.safe_mode == "escape":
            replacements = [
                ('&', '&amp;'),
                ('<', '&lt;'),
                ('>', '&gt;'),
            ]
            for before, after in replacements:
                s = s.replace(before, after)
            return s
        else:
            raise MarkdownError("invalid value for 'safe_mode': %r (must be "
                                "'escape' or 'replace')" % self.safe_mode)

    _tail_of_inline_link_re = re.compile(r'''
          # Match tail of: [text](/url/) or [text](/url/ "title")
          \(            # literal paren
            [ \t]*
            (?P<url>            # \1
                <.*?>
                |
                .*?
            )
            [ \t]*
            (                   # \2
              (['"])            # quote char = \3
              (?P<title>.*?)
              \3                # matching quote
            )?                  # title is optional
          \)
        ''', re.X | re.S)
    _tail_of_reference_link_re = re.compile(r'''
          # Match tail of: [text][id]
          [ ]?          # one optional space
          (?:\n[ ]*)?   # one optional newline followed by spaces
          \[
            (?P<id>.*?)
          \]
        ''', re.X | re.S)

    def _do_links(self, text):
        """Turn Markdown link shortcuts into XHTML <a> and <img> tags.

        This is a combination of Markdown.pl's _DoAnchors() and
        _DoImages(). They are done together because that simplified the
        approach. It was necessary to use a different approach than
        Markdown.pl because of the lack of atomic matching support in
        Python's regex engine used in $g_nested_brackets.
        """
        MAX_LINK_TEXT_SENTINEL = 3000  # markdown2 issue 24

        # `anchor_allowed_pos` is used to support img links inside
        # anchors, but not anchors inside anchors. An anchor's start
        # pos must be `>= anchor_allowed_pos`.
        anchor_allowed_pos = 0

        curr_pos = 0
        while True: # Handle the next link.
            # The next '[' is the start of:
            # - an inline anchor:   [text](url "title")
            # - a reference anchor: [text][id]
            # - an inline img:      ![text](url "title")
            # - a reference img:    ![text][id]
            # - a footnote ref:     [^id]
            #   (Only if 'footnotes' extra enabled)
            # - a footnote defn:    [^id]: ...
            #   (Only if 'footnotes' extra enabled) These have already
            #   been stripped in _strip_footnote_definitions() so no
            #   need to watch for them.
            # - a link definition:  [id]: url "title"
            #   These have already been stripped in
            #   _strip_link_definitions() so no need to watch for them.
            # - not markup:         [...anything else...
            try:
                start_idx = text.index('[', curr_pos)
            except ValueError:
                break
            text_length = len(text)

            # Find the matching closing ']'.
            # Markdown.pl allows *matching* brackets in link text so we
            # will here too. Markdown.pl *doesn't* currently allow
            # matching brackets in img alt text -- we'll differ in that
            # regard.
            bracket_depth = 0
            for p in range(start_idx+1, min(start_idx+MAX_LINK_TEXT_SENTINEL, 
                                            text_length)):
                ch = text[p]
                if ch == ']':
                    bracket_depth -= 1
                    if bracket_depth < 0:
                        break
                elif ch == '[':
                    bracket_depth += 1
            else:
                # Closing bracket not found within sentinel length.
                # This isn't markup.
                curr_pos = start_idx + 1
                continue
            link_text = text[start_idx+1:p]

            # Possibly a footnote ref?
            if "footnotes" in self.extras and link_text.startswith("^"):
                normed_id = re.sub(r'\W', '-', link_text[1:])
                if normed_id in self.footnotes:
                    self.footnote_ids.append(normed_id)
                    result = '<sup class="footnote-ref" id="fnref-%s">' \
                             '<a href="#fn-%s">%s</a></sup>' \
                             % (normed_id, normed_id, len(self.footnote_ids))
                    text = text[:start_idx] + result + text[p+1:]
                else:
                    # This id isn't defined, leave the markup alone.
                    curr_pos = p+1
                continue

            # Now determine what this is by the remainder.
            p += 1
            if p == text_length:
                return text

            # Inline anchor or img?
            if text[p] == '(': # attempt at perf improvement
                match = self._tail_of_inline_link_re.match(text, p)
                if match:
                    # Handle an inline anchor or img.
                    is_img = start_idx > 0 and text[start_idx-1] == "!"
                    if is_img:
                        start_idx -= 1

                    url, title = match.group("url"), match.group("title")
                    if url and url[0] == '<':
                        url = url[1:-1]  # '<url>' -> 'url'
                    # We've got to encode these to avoid conflicting
                    # with italics/bold.
                    url = url.replace('*', self._escape_table['*']) \
                             .replace('_', self._escape_table['_'])
                    if title:
                        title_str = ' title="%s"' \
                            % title.replace('*', self._escape_table['*']) \
                                   .replace('_', self._escape_table['_']) \
                                   .replace('"', '&quot;')
                    else:
                        title_str = ''
                    if is_img:
                        result = '<img src="%s" alt="%s"%s%s' \
                            % (url.replace('"', '&quot;'),
                               link_text.replace('"', '&quot;'),
                               title_str, self.empty_element_suffix)
                        curr_pos = start_idx + len(result)
                        text = text[:start_idx] + result + text[match.end():]
                    elif start_idx >= anchor_allowed_pos:
                        result_head = '<a href="%s"%s>' % (url, title_str)
                        result = '%s%s</a>' % (result_head, link_text)
                        # <img> allowed from curr_pos on, <a> from
                        # anchor_allowed_pos on.
                        curr_pos = start_idx + len(result_head)
                        anchor_allowed_pos = start_idx + len(result)
                        text = text[:start_idx] + result + text[match.end():]
                    else:
                        # Anchor not allowed here.
                        curr_pos = start_idx + 1
                    continue

            # Reference anchor or img?
            else:
                match = self._tail_of_reference_link_re.match(text, p)
                if match:
                    # Handle a reference-style anchor or img.
                    is_img = start_idx > 0 and text[start_idx-1] == "!"
                    if is_img:
                        start_idx -= 1
                    link_id = match.group("id").lower()
                    if not link_id:
                        link_id = link_text.lower()  # for links like [this][]
                    if link_id in self.urls:
                        url = self.urls[link_id]
                        # We've got to encode these to avoid conflicting
                        # with italics/bold.
                        url = url.replace('*', self._escape_table['*']) \
                                 .replace('_', self._escape_table['_'])
                        title = self.titles.get(link_id)
                        if title:
                            title = title.replace('*', self._escape_table['*']) \
                                         .replace('_', self._escape_table['_'])
                            title_str = ' title="%s"' % title
                        else:
                            title_str = ''
                        if is_img:
                            result = '<img src="%s" alt="%s"%s%s' \
                                % (url.replace('"', '&quot;'),
                                   link_text.replace('"', '&quot;'),
                                   title_str, self.empty_element_suffix)
                            curr_pos = start_idx + len(result)
                            text = text[:start_idx] + result + text[match.end():]
                        elif start_idx >= anchor_allowed_pos:
                            result = '<a href="%s"%s>%s</a>' \
                                % (url, title_str, link_text)
                            result_head = '<a href="%s"%s>' % (url, title_str)
                            result = '%s%s</a>' % (result_head, link_text)
                            # <img> allowed from curr_pos on, <a> from
                            # anchor_allowed_pos on.
                            curr_pos = start_idx + len(result_head)
                            anchor_allowed_pos = start_idx + len(result)
                            text = text[:start_idx] + result + text[match.end():]
                        else:
                            # Anchor not allowed here.
                            curr_pos = start_idx + 1
                    else:
                        # This id isn't defined, leave the markup alone.
                        curr_pos = match.end()
                    continue

            # Otherwise, it isn't markup.
            curr_pos = start_idx + 1

        return text 

    def header_id_from_text(self, text, prefix, n):
        """Generate a header id attribute value from the given header
        HTML content.
        
        This is only called if the "header-ids" extra is enabled.
        Subclasses may override this for different header ids.
        
        @param text {str} The text of the header tag
        @param prefix {str} The requested prefix for header ids. This is the
            value of the "header-ids" extra key, if any. Otherwise, None.
        @param n {int} The <hN> tag number, i.e. `1` for an <h1> tag.
        @returns {str} The value for the header tag's "id" attribute. Return
            None to not have an id attribute and to exclude this header from
            the TOC (if the "toc" extra is specified).
        """
        header_id = _slugify(text)
        if prefix and isinstance(prefix, basestring):
            header_id = prefix + '-' + header_id
        if header_id in self._count_from_header_id:
            self._count_from_header_id[header_id] += 1
            header_id += '-%s' % self._count_from_header_id[header_id]
        else:
            self._count_from_header_id[header_id] = 1
        return header_id

    _toc = None
    def _toc_add_entry(self, level, id, name):
        if self._toc is None:
            self._toc = []
        self._toc.append((level, id, name))

    _setext_h_re = re.compile(r'^(.+)[ \t]*\n(=+|-+)[ \t]*\n+', re.M)
    def _setext_h_sub(self, match):
        n = {"=": 1, "-": 2}[match.group(2)[0]]
        demote_headers = self.extras.get("demote-headers")
        if demote_headers:
            n = min(n + demote_headers, 6)
        header_id_attr = ""
        if "header-ids" in self.extras:
            header_id = self.header_id_from_text(match.group(1),
                self.extras["header-ids"], n)
            if header_id:
                header_id_attr = ' id="%s"' % header_id
        html = self._run_span_gamut(match.group(1))
        if "toc" in self.extras and header_id:
            self._toc_add_entry(n, header_id, html)
        return "<h%d%s>%s</h%d>\n\n" % (n, header_id_attr, html, n)

    _atx_h_re = re.compile(r'''
        ^(\#{1,6})  # \1 = string of #'s
        [ \t]*
        (.+?)       # \2 = Header text
        [ \t]*
        (?<!\\)     # ensure not an escaped trailing '#'
        \#*         # optional closing #'s (not counted)
        \n+
        ''', re.X | re.M)
    def _atx_h_sub(self, match):
        n = len(match.group(1))
        demote_headers = self.extras.get("demote-headers")
        if demote_headers:
            n = min(n + demote_headers, 6)
        header_id_attr = ""
        if "header-ids" in self.extras:
            header_id = self.header_id_from_text(match.group(2),
                self.extras["header-ids"], n)
            if header_id:
                header_id_attr = ' id="%s"' % header_id
        html = self._run_span_gamut(match.group(2))
        if "toc" in self.extras and header_id:
            self._toc_add_entry(n, header_id, html)
        return "<h%d%s>%s</h%d>\n\n" % (n, header_id_attr, html, n)

    def _do_headers(self, text):
        # Setext-style headers:
        #     Header 1
        #     ========
        #  
        #     Header 2
        #     --------
        text = self._setext_h_re.sub(self._setext_h_sub, text)

        # atx-style headers:
        #   # Header 1
        #   ## Header 2
        #   ## Header 2 with closing hashes ##
        #   ...
        #   ###### Header 6
        text = self._atx_h_re.sub(self._atx_h_sub, text)

        return text


    _marker_ul_chars  = '*+-'
    _marker_any = r'(?:[%s]|\d+\.)' % _marker_ul_chars
    _marker_ul = '(?:[%s])' % _marker_ul_chars
    _marker_ol = r'(?:\d+\.)'

    def _list_sub(self, match):
        lst = match.group(1)
        lst_type = match.group(3) in self._marker_ul_chars and "ul" or "ol"
        result = self._process_list_items(lst)
        if self.list_level:
            return "<%s>\n%s</%s>\n" % (lst_type, result, lst_type)
        else:
            return "<%s>\n%s</%s>\n\n" % (lst_type, result, lst_type)

    def _do_lists(self, text):
        # Form HTML ordered (numbered) and unordered (bulleted) lists.

        for marker_pat in (self._marker_ul, self._marker_ol):
            # Re-usable pattern to match any entire ul or ol list:
            less_than_tab = self.tab_width - 1
            whole_list = r'''
                (                   # \1 = whole list
                  (                 # \2
                    [ ]{0,%d}
                    (%s)            # \3 = first list item marker
                    [ \t]+
                  )
                  (?:.+?)
                  (                 # \4
                      \Z
                    |
                      \n{2,}
                      (?=\S)
                      (?!           # Negative lookahead for another list item marker
                        [ \t]*
                        %s[ \t]+
                      )
                  )
                )
            ''' % (less_than_tab, marker_pat, marker_pat)
        
            # We use a different prefix before nested lists than top-level lists.
            # See extended comment in _process_list_items().
            #
            # Note: There's a bit of duplication here. My original implementation
            # created a scalar regex pattern as the conditional result of the test on
            # $g_list_level, and then only ran the $text =~ s{...}{...}egmx
            # substitution once, using the scalar as the pattern. This worked,
            # everywhere except when running under MT on my hosting account at Pair
            # Networks. There, this caused all rebuilds to be killed by the reaper (or
            # perhaps they crashed, but that seems incredibly unlikely given that the
            # same script on the same server ran fine *except* under MT. I've spent
            # more time trying to figure out why this is happening than I'd like to
            # admit. My only guess, backed up by the fact that this workaround works,
            # is that Perl optimizes the su…