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Wirth syntax notation
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<p> Wirth syntax notation (WSN) is a <a href="/facts/Metasyntax/Mmb3tj6n">metasyntax</a>, that is, a formal way to describe <a href="/facts/Formal_language/crDTyP8q">formal languages</a>. Originally proposed by <a href="/facts/Niklaus_Wirth/krsLUB11">Niklaus Wirth</a> in 1977 as an alternative to <a href="/facts/Backus%E2%80%93Naur_form/xbMXDntc">Backus–Naur form</a> (BNF). It has several advantages over BNF in that it contains an explicit iteration construct, and it avoids the use of an explicit symbol for the empty string (such as <empty> or ε). </p><p>WSN has been used in several <a href="/facts/International_Organization_for_Standardization/IacYR7Yg">international standards</a>, starting with <a href="/facts/ISO_10303-21/WlGoNt7m">ISO 10303-21</a>. It was also used to define the syntax of <a href="/facts/ISO_10303-11/lAmytNRX">EXPRESS</a>, the <a href="/facts/Data_modelling/bryA2moP">data modelling</a> language of <a href="/facts/ISO_10303/1sUvrP4t">STEP</a>. </p>