Feat: conceal backslash if it is a valid escape code leader

This should close preservim#510

Author: flw-cn

syntax/markdown.vim

@@ -86,12 +86,12 @@ syn region mkdLinkTitle matchgroup=mkdDelimiter start=+'+     end=+'+  contained
 syn region mkdLinkTitle matchgroup=mkdDelimiter start=+(+     end=+)+  contained
 
 "HTML headings
-syn region htmlH1       matchgroup=mkdHeading     start="^\s*#"                   end="$" contains=mkdLink,mkdInlineURL,@Spell
-syn region htmlH2       matchgroup=mkdHeading     start="^\s*##"                  end="$" contains=mkdLink,mkdInlineURL,@Spell
-syn region htmlH3       matchgroup=mkdHeading     start="^\s*###"                 end="$" contains=mkdLink,mkdInlineURL,@Spell
-syn region htmlH4       matchgroup=mkdHeading     start="^\s*####"                end="$" contains=mkdLink,mkdInlineURL,@Spell
-syn region htmlH5       matchgroup=mkdHeading     start="^\s*#####"               end="$" contains=mkdLink,mkdInlineURL,@Spell
-syn region htmlH6       matchgroup=mkdHeading     start="^\s*######"              end="$" contains=mkdLink,mkdInlineURL,@Spell
+syn region htmlH1       matchgroup=mkdHeading     start="^\s*#"                   end="$" contains=mkdEscape,mkdLink,mkdInlineURL,@Spell
+syn region htmlH2       matchgroup=mkdHeading     start="^\s*##"                  end="$" contains=mkdEscape,mkdLink,mkdInlineURL,@Spell
+syn region htmlH3       matchgroup=mkdHeading     start="^\s*###"                 end="$" contains=mkdEscape,mkdLink,mkdInlineURL,@Spell
+syn region htmlH4       matchgroup=mkdHeading     start="^\s*####"                end="$" contains=mkdEscape,mkdLink,mkdInlineURL,@Spell
+syn region htmlH5       matchgroup=mkdHeading     start="^\s*#####"               end="$" contains=mkdEscape,mkdLink,mkdInlineURL,@Spell
+syn region htmlH6       matchgroup=mkdHeading     start="^\s*######"              end="$" contains=mkdEscape,mkdLink,mkdInlineURL,@Spell
 syn match  htmlH1       /^.\+\n=\+$/ contains=mkdLink,mkdInlineURL,@Spell
 syn match  htmlH2       /^.\+\n-\+$/ contains=mkdLink,mkdInlineURL,@Spell
 
@@ -116,6 +116,9 @@ syn match  mkdRule         /^\s*\*\s\{0,1}\*\s\{0,1}\*\(\*\|\s\)*$/
 syn match  mkdRule         /^\s*-\s\{0,1}-\s\{0,1}-\(-\|\s\)*$/
 syn match  mkdRule         /^\s*_\s\{0,1}_\s\{0,1}_\(_\|\s\)*$/
 
+syntax region mkdEscape matchgroup=mkdEscape start=/\\\ze[\\\x60*{}\[\]()#+\-,.!_>~|"$%&'\/:;<=?@^]/ end=/.\zs/ keepend contains=mkdEscapeCh contained oneline concealends
+syntax match mkdEscapeCh /./ contained
+
 " YAML frontmatter
 if get(g:, 'vim_markdown_frontmatter', 0)
   syn include @yamlTop syntax/yaml.vim
@@ -155,7 +158,7 @@ if get(g:, 'vim_markdown_strikethrough', 0)
     HtmlHiLink mkdStrike        htmlStrike
 endif
 
-syn cluster mkdNonListItem contains=@htmlTop,htmlItalic,htmlBold,htmlBoldItalic,mkdFootnotes,mkdInlineURL,mkdLink,mkdLinkDef,mkdLineBreak,mkdBlockquote,mkdCode,mkdRule,htmlH1,htmlH2,htmlH3,htmlH4,htmlH5,htmlH6,mkdMath,mkdStrike
+syn cluster mkdNonListItem contains=@htmlTop,htmlItalic,htmlBold,htmlBoldItalic,mkdFootnotes,mkdInlineURL,mkdLink,mkdLinkDef,mkdLineBreak,mkdBlockquote,mkdCode,mkdRule,htmlH1,htmlH2,htmlH3,htmlH4,htmlH5,htmlH6,mkdMath,mkdStrike,mkdEscape
 
 "highlighting for Markdown groups
 HtmlHiLink mkdString        String
@@ -177,6 +180,7 @@ HtmlHiLink mkdLinkDef       mkdID
 HtmlHiLink mkdLinkDefTarget mkdURL
 HtmlHiLink mkdLinkTitle     htmlString
 HtmlHiLink mkdDelimiter     Delimiter
+HtmlHiLink mkdEscape        Special
 
 let b:current_syntax = "mkd"
 

test/escapes.md

@@ -0,0 +1,7 @@
+Escapes:
+
+text: \\ \` \* \{ \} \[ \] \( \) \# \+ \- \.  \!  \_ \< \> \~ \| \" \$ \% \& \' \/ \: \; \= \?  \@ \^
+
+## Header: \\ \` \* \{ \} \[ \] \( \) \# \+ \- \.  \!  \_ \< \> \~ \| \" \$ \% \& \' \/ \: \; \= \?  \@ \^
+
+Non Escapes: \a \b \c \f \n \r \t \v \x \o