Marcella Anselmo Dora Giammarresi Maria Madonia Regular Expressions for Two-Dimensional Languages Over One-Letter Alphabet. 63-75 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3340&spage=63 2004 conf/dlt/2004 Developments in Language Theory db/conf/dlt/dlt2004.html#AnselmoGM04