grex: add command (#4396)

pages/common/grex.md

@@ -0,0 +1,28 @@
+# grex
+
+> Simple command line tool to generate regular expressions.
+> More information: <https://github.com/pemistahl/grex>.
+
+- Generate a simple regular expression:
+
+` grex {{space_separated_strings}}`
+
+- Generate a case-insensitive regex:
+
+`grex -i {{space_separated_strings}}`
+
+- Replace digits with '\d':
+
+`grex -d {{space_separated_strings}}`
+
+- Replace unicode word character with '\w':
+
+`grex -w {{space_separated_strings}}`
+
+- Replace spaces with '\s':
+
+`grex -s {{space_separated_strings}}`
+
+- Add {min, max} quantifier representation for repeating sub-strings:
+
+`grex -r {{space_separated_strings}}`