present: allow markup inside punctuation connected to other text

This allows markup bracketed by punctuation even when the
punctuation has text on the other side, like in:

- Markup—_especially_italic_text_—can easily be overused.
- We want to increase `go`vet`'s usage.

Change-Id: I0c6ca790f23f705d8c8ba8a225c0280b916ebb6c
Reviewed-on: https://go-review.googlesource.com/33662
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

present/doc.go

@@ -77,14 +77,22 @@ Fonts:
 Within the input for plain text or lists, text bracketed by font
 markers will be presented in italic, bold, or program font.
 Marker characters are _ (italic), * (bold) and ` (program font).
-Unmatched markers appear as plain text.
-Within marked text, a single marker character becomes a space
-and a doubled single marker quotes the marker character.
+An opening marker must be preceded by a space or punctuation
+character or else be at start of a line; similarly, a closing
+marker must be followed by a space or punctuation character or
+else be at the end of a line. Unmatched markers appear as plain text.
+There must be no spaces between markers. Within marked text,
+a single marker character becomes a space and a doubled single
+marker quotes the marker character.
+
+at the beginning of a line or
+else be preceded by a space or punctuation; similarly a closing
+marker must be at the end of the lineo
 
 	_italic_
 	*bold*
 	`program`
-	_this_is_all_italic_
+	Markup—_especially_italic_text_—can easily be overused.
 	_Why_use_scoped__ptr_? Use plain ***ptr* instead.
 
 Inline links:

present/style.go

@@ -52,16 +52,16 @@ Word:
 			words[w] = link
 			continue Word
 		}
-		const punctuation = `.,;:()!?—–'"`
 		const marker = "_*`"
 		// Initial punctuation is OK but must be peeled off.
 		first := strings.IndexAny(word, marker)
 		if first == -1 {
 			continue Word
 		}
-		// Is the marker prefixed only by punctuation?
-		for _, r := range word[:first] {
-			if !strings.ContainsRune(punctuation, r) {
+		// Opening marker must be at the beginning of the token or else preceded by punctuation.
+		if first != 0 {
+			r, _ := utf8.DecodeLastRuneInString(word[:first])
+			if !unicode.IsPunct(r) {
 				continue Word
 			}
 		}
@@ -81,17 +81,18 @@ Word:
 			open += "<code>"
 			close = "</code>"
 		}
-		// Terminal punctuation is OK but must be peeled off.
+		// Closing marker must be at the end of the token or else followed by punctuation.
 		last := strings.LastIndex(word, word[:1])
 		if last == 0 {
 			continue Word
 		}
-		head, tail := word[:last+1], word[last+1:]
-		for _, r := range tail {
-			if !strings.ContainsRune(punctuation, r) {
+		if last+1 != len(word) {
+			r, _ := utf8.DecodeRuneInString(word[last+1:])
+			if !unicode.IsPunct(r) {
 				continue Word
 			}
 		}
+		head, tail := word[:last+1], word[last+1:]
 		b.Reset()
 		b.WriteString(open)
 		var wid int

present/style_test.go

@@ -72,6 +72,14 @@ func TestFont(t *testing.T) {
 			`Visit <a href="http://golang.org" target="_blank">golang.org</a> now`},
 		{"my talk ([[http://talks.golang.org/][slides here]])",
 			`my talk (<a href="http://talks.golang.org/" target="_blank">slides here</a>)`},
+		{"Markup—_especially_italic_text_—can easily be overused.",
+			`Markup—<i>especially italic text</i>—can easily be overused.`},
+		{"`go`get`'s codebase", // ascii U+0027 ' before s
+			`<code>go get</code>'s codebase`},
+		{"`go`get`’s codebase", // unicode right single quote U+2019 ’ before s
+			`<code>go get</code>’s codebase`},
+		{"a_variable_name",
+			`a_variable_name`},
 	}
 	for _, test := range tests {
 		out := font(test.in)