Commit
2da7c3f21e2b70cfd08d0f193eeaa6f00e9eb1b8 Parent
ba8f2c48b700a466a8cf7097d5f72d43372d4c37 Author
John MacFarlane <jgm@berkeley.edu >
Date
Sun, 14 Dec 2014 18:21:04 -0800
Improved rules for emphasis and strong emphasis.
This improves parsing of emphasis around punctuation.
Background:
http://talk.commonmark.org/t/emphasis-inside-strong-broken-in-js-implementation-when-parenthesis-involved/903/6
The basic idea of the change is that if the delimiter is part of
a delimiter clump that has punctuation to the left and a normal
character (non-space, non-punctuation) to the right, it can only
be an opener. If it has punctuation to the right and a normal
character (non-space, non-punctuation) to the left, it can only be a closer.
This handles cases like
**Gomphocarpus (*Gomphocarpus physocarpus*, syn. *Asclepias
physocarpa*)**
and
**foo "*bar*" foo**
better than before.
The spec section on Emphasis and Strong Emphasis has been extensively
revised. The C and JS implementations have been brought up to date,
and all tests pass.
diff --git a/spec.txt b/spec.txt
@@ -4390,36 +4390,107 @@ internal emphasis: foo*bar*baz
no emphasis: foo_bar_baz
```
-The following rules capture all of these patterns, while allowing
-for efficient parsing strategies that do not backtrack:
+The rules given below capture all of these patterns, while allowing
+for efficient parsing strategies that do not backtrack.
+
+First, some definitions. A [delimiter run](@delimiter-run) is either
+a sequence of one or more `*` characters that is not preceded or
+followed by a `*` character, or a sequence of one or more `_`
+characters that is not preceded or followed by a `_` character.
+
+A [left-flanking delimiter run](@right-facing-delimiter-run) is
+a [delimiter run](#delimiter-run) that is (a) not followed by [unicode
+whitespace](#unicode-whitespace), and (b) either not followed by a
+[punctuation character](#punctuation-character), or
+preceded by [unicode whitespace](#unicode-whitespace) or
+a [punctuation character](#punctuation-character).
+
+A [right-flanking delimiter run](@left-facing-delimiter-run) is
+a [delimiter run](#delimiter-run) that is (a) not preceded by [unicode
+whitespace](#unicode-whitespace), and (b) either not preceded by a
+[punctuation character](#punctuation-character), or
+followed by [unicode whitespace](#unicode-whitespace) or
+a [punctuation character](#punctuation-character).
+
+Here are some examples of delimiter runs.
+
+ - left-flanking but not right-flanking:
+
+ ```
+ ***abc
+ _abc
+ **"abc"
+ _"abc"
+ ```
+
+ - right-flanking but not left-flanking:
+
+ ```
+ abc***
+ abc_
+ "abc"**
+ _"abc"
+ ```
+
+ - Both right and right-flanking:
+
+ ```
+ abc***def
+ "abc"_"def"
+ ```
+
+ - Neither right nor right-flanking:
+
+ ```
+ abc *** def
+ a _ b
+ ```
+
+(The idea of distinguishing left-flanking and right-flanking
+delimiter runs based on the character before and the character
+after comes from Roopesh Chander's
+[vfmd](http://www.vfmd.org/vfmd-spec/specification/#procedure-for-identifying-emphasis-tags).
+vfmd uses the terminology "emphasis indicator string" instead of "delimiter
+run," and its rules for distinguishing left- and right-flanking runs
+are a bit more complex than the ones given here.)
+
+The following rules define emphasis and strong emphasis:
1. A single `*` character [can open emphasis](@can-open-emphasis)
- iff it is not followed by [unicode whitespace](#unicode-whitespace).
+ iff it is part of a
+ [left-flanking delimiter run](#right-facing-delimiter-run).
2. A single `_` character [can open emphasis](#can-open-emphasis) iff
- it is not followed by [unicode whitespace](#unicode-whitespace)
- and it is not preceded by an ASCII alphanumeric character.
+ it is part of a
+ [left-flanking delimiter run](#right-facing-delimiter-run)
+ and is not preceded by an ASCII alphanumeric character.
3. A single `*` character [can close emphasis](@can-close-emphasis)
- iff it is not preceded by [unicode whitespace](#unicode-whitespace).
+ iff it is part of a
+ [left-flanking delimiter run](#right-facing-delimiter-run).
-4. A single `_` character [can close emphasis](#can-close-emphasis) iff
- it is not preceded by [unicode whitespace](#unicode-whitespace)
+4. A single `_` character [can close emphasis](#can-close-emphasis)
+ iff it is part of a
+ [left-flanking delimiter run](#right-facing-delimiter-run).
and it is not followed by an ASCII alphanumeric character.
5. A double `**` [can open strong emphasis](@can-open-strong-emphasis)
- iff it is not followed by [unicode whitespace](#unicode-whitespace).
+ iff it is part of a
+ [left-flanking delimiter run](#right-facing-delimiter-run).
6. A double `__` [can open strong emphasis](#can-open-strong-emphasis)
- iff it is not followed by [unicode whitespace](#unicode-whitespace)
- and it is not preceded by an ASCII alphanumeric character.
+ iff it is part of a
+ [left-flanking delimiter run](#right-facing-delimiter-run)
+ and is not preceded by an ASCII alphanumeric character.
7. A double `**` [can close strong emphasis](@can-close-strong-emphasis)
- iff it is not preceded by [unicode whitespace](#unicode-whitespace).
+ iff it is part of a
+ [right-flanking delimiter run](#right-facing-delimiter-run).
8. A double `__` [can close strong emphasis](#can-close-strong-emphasis)
- iff it is not preceded by [unicode whitespace](#unicode-whitespace)
- and it is not followed by an ASCII alphanumeric character.
+ iff it is part of a
+ [right-flanking delimiter run](#right-facing-delimiter-run).
+ and is not followed by an ASCII alphanumeric character.
9. Emphasis begins with a delimiter that [can open
emphasis](#can-open-emphasis) and ends with a delimiter that [can close
@@ -4487,7 +4558,8 @@ Rule 1:
.
This is not emphasis, because the opening `*` is followed by
-whitespace:
+whitespace, and hence not part of a [left-flanking delimiter
+run](#right-facing-delimiter-run):
.
a * foo bar*
@@ -4495,6 +4567,16 @@ a * foo bar*
<p>a * foo bar*</p>
.
+This is not emphasis, because the opening `*` is preceded
+by an alphanumeric and followed by punctuation, and hence
+not part of a [left-flanking delimiter run](#right-facing-delimiter-run):
+
+.
+a*"foo"*
+.
+<p>a*"foo"*</p>
+.
+
Unicode nonbreaking spaces count as whitespace, too:
.
@@ -4534,6 +4616,15 @@ _ foo bar_
<p>_ foo bar_</p>
.
+This is not emphasis, because the opening `_` is preceded
+by an alphanumeric and followed by punctuation:
+
+.
+a_"foo"_
+.
+<p>a_"foo"_</p>
+.
+
Emphasis with `_` is not allowed inside ASCII words:
.
@@ -4558,6 +4649,15 @@ But it is permitted inside non-ASCII words:
Rule 3:
+This is not emphasis, because the closing delimiter does
+not match the opening delimiter:
+
+.
+_foo*
+.
+<p>_foo*</p>
+.
+
This is not emphasis, because the closing `*` is preceded by
whitespace:
@@ -4567,6 +4667,26 @@ whitespace:
<p>*foo bar *</p>
.
+This is not emphasis, because the second `*` is
+preceded by punctuation and followed by an alphanumeric
+(hence it is not part of a [right-flanking delimiter
+run](#left-facing-delimiter-run):
+
+.
+*(*foo)
+.
+<p>*(*foo)</p>
+.
+
+The point of this restriction is more easily appreciated
+with this example:
+
+.
+*(*foo*)*
+.
+<p><em>(<em>foo</em>)</em></p>
+.
+
Intraword emphasis with `*` is allowed:
.
@@ -4587,7 +4707,24 @@ _foo bar _
<p>_foo bar _</p>
.
-Intraword emphasis:
+This is not emphasis, because the second `_` is
+preceded by punctuation and followed by an alphanumeric:
+
+.
+_(_foo)
+.
+<p>_(_foo)</p>
+.
+
+This is emphasis within emphasis:
+
+.
+_(_foo_)_
+.
+<p><em>(<em>foo</em>)</em></p>
+.
+
+Intraword emphasis is disallowed for `_`:
.
_foo_bar
@@ -4624,6 +4761,16 @@ followed by whitespace:
<p>** foo bar**</p>
.
+This is not strong emphasis, because the opening `**` is preceded
+by an alphanumeric and followed by punctuation, and hence
+not part of a [left-flanking delimiter run](#right-facing-delimiter-run):
+
+.
+a**"foo"**
+.
+<p>a**"foo"**</p>
+.
+
Intraword strong emphasis with `**` is permitted:
.
@@ -4649,7 +4796,16 @@ __ foo bar__
<p>__ foo bar__</p>
.
-Intraword emphasis examples:
+This is not strong emphasis, because the opening `__` is preceded
+by an alphanumeric and followed by punctuation:
+
+.
+a__"foo"__
+.
+<p>a__"foo"__</p>
+.
+
+Intraword strong emphasis is forbidden with `__`:
.
foo__bar__
@@ -4689,6 +4845,38 @@ by whitespace:
(Nor can it be interpreted as an emphasized `*foo bar *`, because of
Rule 11.)
+This is not strong emphasis, because the second `**` is
+preceded by punctuation and followed by an alphanumeric:
+
+.
+**(**foo)
+.
+<p>**(**foo)</p>
+.
+
+The point of this restriction is more easily appreciated
+with these examples:
+
+.
+*(**foo**)*
+.
+<p><em>(<strong>foo</strong>)</em></p>
+.
+
+.
+**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
+*Asclepias physocarpa*)**
+.
+<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn.
+<em>Asclepias physocarpa</em>)</strong></p>
+.
+
+.
+**foo "*bar*" foo**
+.
+<p><strong>foo "<em>bar</em>" foo</strong></p>
+.
+
Intraword emphasis:
.
@@ -4708,7 +4896,25 @@ __foo bar __
<p>__foo bar __</p>
.
-Intraword strong emphasis examples:
+This is not strong emphasis, because the second `__` is
+preceded by punctuation and followed by an alphanumeric:
+
+.
+__(__foo)
+.
+<p>__(__foo)</p>
+.
+
+The point of this restriction is more easily appreciated
+with this example:
+
+.
+_(__foo__)_
+.
+<p><em>(<strong>foo</strong>)</em></p>
+.
+
+Intraword strong emphasis is forbidden with `__`:
.
__foo__bar