// Please enter a detailed description of the issue // Description of the issue:

When creating an ATX-style heading, if the header text is enclosed inside of square brackets, then the heading is not rendered correctly. The hash marks are displayed as-is.

// Please enter a list of steps required to reproduce the issue // Steps to reproduce:

1. Create an ATX-style heading.
   
2. Add text on the same line, with a single space between the hash marks and the words. Notice that Marked renders the heading as expected.
   
3. Enclose the text inside of square brackets. Notice that Marked no longer renders this line as a heading, and the hash marks are now visible.

// If applicable please attach a sample document that causes the issue for you.

// Configuration (do not edit) Marked 2 1022 (Mac App Store)

macOS Version: 11.5.1

additionalMathJaxConfig:
allowPageBreakInCode: YES
bookTxtIsLeanpub: NO
summaryMdIsGitBook: NO
codeIsPoetry: NO
codeCanWrap: YES
collapsibleHeadlines: NO
convertGithubCheckboxes: YES
convertGithubEmoji: YES
convertYAMLToMMD: NO
defaultMathJaxConfig: TeX-AMS-MML_HTMLorMML
defaultPreviewStyle: 3
defaultProcessor: MultiMarkdown
enableMiniMap: YES
excludeBlockQuotesFromCounts: NO
excludeFootnotesFromCounts: NO
excludeVerbatimFromCounts: NO
excludeCaptionsFromCounts: NO
folderPreviewExtensionsArray: md, markdown, txt, mdown, html, opml
githubNewlines: YES
h1IsPageBreak: NO
h2IsPageBreak: NO
hrIsPageBreak: NO
includeMathJax: NO
includeKatex: NO
limitTextWidth: NO
mathNumberEquations: NO
mathNumberEquationsSide: 0
mathNumberEquationsAMSOnly: NO
minimalJavaScript: NO
outlineModeExtensions:
permissionGranted: NO
printHeaderBottomLeft:
printHeaderBottomRight:
printHeaderFirstPage: NO
printFooterFirstPage: NO
printHeaderTopLeft:
printHeaderTopRight:
processHTMLFiles: YES
removeMetadataForPreview: NO
removeYAMLHeaders: NO
scrollToEdit: YES
shouldDetectCritic: YES
shouldHighlightMarkupErrors: NO
shouldLoadCritic: NO
syntaxHighlight: YES
syntaxHighlightOnlyLang: NO
useCustomMarkdownProcessor: NO
customMarkdownProcessor:
useCustomPreprocessor: NO
customPreprocessor:
useFirstH1AsPrintTitle: YES