Reference category

Text & patterns

Matching text and presenting text are different problems that share the same input. Regular expressions describe a set of strings and are interpreted by an engine whose syntax and backtracking behaviour vary; Markdown describes structure in plain text and is interpreted by a renderer whose supported subset varies. In both cases the spec you assume is part of the bug.

These entries document the constructs that matter in daily work, name the engine or flavour when behaviour differs, and show the failure modes — catastrophic backtracking, greedy matches, escaped HTML, tables that only some renderers support. The paired tools render or test the input directly.

Reference sets in this category (2)

Regular Expression Reference

How regular expression syntax works in JavaScript and common engines: character classes, quantifiers, anchors, groups, lookaround, backreferences, flags and backtracking. Facts from the ECMAScript and PCRE specifications, paired with a regex tester.

10 entries →

Markdown Reference

Markdown syntax with the edge cases that actually bite: headings, paragraphs, emphasis, links, images, lists, code, blockquotes, tables and task lists. Facts from CommonMark and GitHub Flavored Markdown, paired with a live previewer.

10 entries →

All 20 entries

Regular Expression Reference 10 entries

Character classesSquare brackets match one character from a set, with ranges, negation and shorthand classes such as \d and \w. QuantifiersQuantifiers repeat the previous atom: * zero or more, + one or more, ? optional, and {n,m} an explicit count. Greedy and lazy matchingGreedy quantifiers take as much as possible; adding ? makes them lazy and take as little as possible. Anchors and word boundariesAnchors match positions, not characters: ^ and $ for line or string edges, \b for a word boundary. Capturing groupsParentheses group a subpattern and capture its text; groups are numbered by the order of their opening parenthesis. Non-capturing groups(?:...) groups a subpattern for quantifiers and alternation without creating a capture. Lookahead and lookbehindLookaround asserts what comes before or after a position without consuming characters. BackreferencesA backreference matches the same text captured earlier by a group, such as \1 or \k<name>. Regex flagsFlags change how a pattern is interpreted: global, case-insensitive, multiline, dot-all, Unicode, sticky and indices. Catastrophic backtrackingNested quantifiers can make a regex take exponential time, a failure mode known as ReDoS.

Markdown Reference 10 entries

HeadingsATX headings use one to six # characters; setext headings underline text with = or -. Paragraphs and line breaksA blank line starts a new paragraph; a single newline is usually just a space. EmphasisAsterisks and underscores mark italic and bold text, with rules about word boundaries. LinksInline, reference and automatic links connect text to a URL. ImagesAn image is a link with a leading ! and the alt text in square brackets. ListsOrdered and unordered lists use markers at the start of a line, with indentation for nesting. CodeBackticks mark inline code; triple backticks create fenced blocks with optional language info. BlockquotesA > marker at the start of a line creates a blockquote; markers nest. TablesGFM tables use pipes and a delimiter row to define columns and alignment. Task listsGFM task lists add checkboxes to list items with [ ] and [x].

Reference sets in this category (2)

Regular Expression Reference

Markdown Reference

All 20 entries

Regular Expression Reference 10 entries

Markdown Reference 10 entries

Tools that work with these entries

Regex Tester

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Markdown Preview

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