lex: add page (#1783)

pages/common/lex.md

@@ -0,0 +1,16 @@
+# lex
+
+> Lexical analyser generator.
+> Given the specification for a lexical analyser, generates C code implementing it.
+
+- Generate an analyser from a Lex file:
+
+`lex {{analyser.l}}`
+
+- Specify the output file:
+
+`lex {{analyser.l}} --outfile {{analyser.c}}`
+
+- Compile a C file generated by Lex:
+
+`cc {{path/to/lex.yy.c}} --output {{executable}}`