Fix go-gitea#27605: inline math blocks can't be preceeded/followed by…

… alphanumerical characters

Inline math blocks couldn't be preceeded or succeeded by alphanumerical characters due to changes introduced in PR go-gitea#21171. Removed the condition that caused this (precedingCharacter) and added a new if statement that checks if a specific '$' was escaped using '\'.
Signed-off-by: João Tiago <joao.leal.tintas@tecnico.ulisboa.pt>

modules/markup/markdown/markdown_test.go

@@ -509,15 +509,15 @@ func TestMathBlock(t *testing.T) {
  },
  {
  `$a a$b b$`,
- `<p><code class="language-math is-loading">a a$b b</code></p>` + nl,
+ `<p><code class="language-math is-loading">a a</code>b b$</p>` + nl,
  },
  {
  `a a$b b`,
  `<p>a a$b b</p>` + nl,
  },
  {
  `a$b $a a$b b$`,
- `<p>a$b <code class="language-math is-loading">a a$b b</code></p>` + nl,
+ `<p>a<code class="language-math is-loading">b </code>a a<code class="language-math is-loading">b b</code></p>` + nl,
  },
  {
  "$$a$$",

modules/markup/markdown/math/inline_parser.go

@@ -55,12 +55,6 @@ func (parser *inlineParser) Parse(parent ast.Node, block text.Reader, pc parser.
  return nil
  }
 
- precedingCharacter := block.PrecendingCharacter()
- if precedingCharacter < 256 && isAlphanumeric(byte(precedingCharacter)) {
- // need to exclude things like `a$` from being considered a start
- return nil
- }
-
  // move the opener marker point at the start of the text
  opener := len(parser.start)
 
@@ -75,14 +69,15 @@ func (parser *inlineParser) Parse(parent ast.Node, block text.Reader, pc parser.
  ender += pos
 
  // Now we want to check the character at the end of our parser section
- // that is ender + len(parser.end)
+ // that is ender + len(parser.end) and check if char before ender is '\'
  pos = ender + len(parser.end)
  if len(line) <= pos {
  break
  }
- if !isAlphanumeric(line[pos]) {
+ if  line[ender-1] != '\\' {
  break
  }
+
  // move the pointer onwards
  ender += len(parser.end)
  }