顯示文字內的子字串

				
					{"blocks":"<xml xmlns=\"https://developers.google.com/blockly/xml\"><variables><variable>mainLayout</variable><variable>mainScreen</variable><variable>text</variable></variables><block type=\"pxt-on-start\" x=\"0\" y=\"0\"><statement name=\"HANDLER\"><block type=\"variables_set\"><field name=\"VAR\">mainLayout</field><value name=\"VALUE\"><shadow type=\"math_number\"><field name=\"NUM\">0</field></shadow><block type=\"ScreenItem_createLayout\"/></value><next><block type=\"variables_set\"><field name=\"VAR\">mainScreen</field><value name=\"VALUE\"><shadow type=\"math_number\"><field name=\"NUM\">0</field></shadow><block type=\"Screen_create\"><value name=\"label\"><shadow type=\"text\"><field name=\"TEXT\">Main</field></shadow></value><value name=\"layout\"><block type=\"variables_get\"><field name=\"VAR\">mainLayout</field></block></value></block></value><next><block type=\"variables_set\"><field name=\"VAR\">text</field><value name=\"VALUE\"><shadow type=\"math_number\"><field name=\"NUM\">0</field></shadow><block type=\"text\"><field name=\"TEXT\">Hello World!</field></block></value><next><block type=\"ScreenItem_Layout_add\"><value name=\"this\"><block type=\"variables_get\"><field name=\"VAR\">mainLayout</field></block></value><value name=\"item\"><block type=\"Control_createTextBlock\"><mutation xmlns=\"http://www.w3.org/1999/xhtml\" _expanded=\"1\" _input_init=\"true\"></mutation><value name=\"text\"><block type=\"string_substr\"><value name=\"this\"><shadow type=\"text\"><field name=\"TEXT\"/></shadow><block type=\"variables_get\"><field name=\"VAR\">text</field></block></value><value name=\"start\"><shadow type=\"math_number\"><field name=\"NUM\">0</field></shadow></value><value name=\"length\"><shadow type=\"math_number\"><field name=\"NUM\">7</field></shadow></value></block></value></block></value></block></next></block></next></block></next></block></statement></block></xml>"}