coreutils commands: move pages to common/ folder (#2442)

pages/common/expr.md

@@ -0,0 +1,23 @@
+# expr
+
+> Evaluate expressions and manipulate strings.
+
+- Get string length:
+
+`expr length {{string}}`
+
+- Evaluate logical or math expression with an operator ('+', '-', '*', '&', '|', etc.). Special symbols should be escaped:
+
+`expr {{first_argument}} {{operator}} {{second_argument}}`
+
+- Get position of the first character in 'string' that matches 'substring':
+
+`echo $(expr index {{string}} {{substring}})`
+
+- Extract part of the string:
+
+`echo $(expr substr {{string}} {{position_to_start}} {{number_of_characters}}`
+
+- Extract part of the string which matches a regular expression:
+
+`echo $(expr {{string}} : '\({{regular_expression}}\)')`