# Regular Expression A regular expression is a compact pattern language for recognizing and capturing structured character sequences. Atomic Markdown's reference parser uses one regular expression to capture the document opening, quotation opening, shared text, and quotation ending. The unified rule handles identity, capitalization, lexical reconstruction, substantive endings, and mid-quote forms without branching. Related: [[wiki/Atomic Markdown|Atomic Markdown]], [[wiki/Deterministic Transformation|Deterministic Transformation]], [[wiki/Protected Region|Protected Region]], [[wiki/Dual-Surface Validation|Dual-Surface Validation]].