flex: add page (#1848)

pages/common/flex.md

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