Add some docs on lex syntax

doc/src/SUMMARY.md

@@ -7,6 +7,7 @@
     - [Extensions](lexextensions.md)
   - [Hand-written lexers](manuallexer.md)
   - [Start States](start_states.md)
+  - [Syntax](lexsyntax.md)
 - [Parsing](parsing.md)
   - [Yacc compatibility](yacccompatibility.md)
     - [Extensions](yaccextensions.md)

doc/src/lexsyntax.md

@@ -0,0 +1,33 @@
+The syntax of the lrlex `.l` format, aims to be familiar to the format used by posix lex.
+It uses the same basic structure as lex.
+
+```
+Definitions
+%%
+Rules
+%%
+User Subroutines
+```
+
+## Definitions
+
+Within the definitions section, you can add an option `%grmtools` section
+documented in [extensions](./lexextensions.md)
+
+## Rules 
+
+Each rule is given by the following elements in sequence:
+
+1. Optional Start State, a name given between angle brackets for example `<Start_State>` documented in [Start States](./start_states.md)
+2. Regex, syntax defined by the rust [regex](https://docs.rs/regex) crate with optional escaping for any character.
+3. Separator space, any horizontal space character in the unicode `Pattern_White_Space` character set
+4. Optional State operator, given between angle brackets documented in [Start States](./start_states.md)
+    + `<+STATE>` Push the state given by `STATE` to the top of the stack.
+    + `<-STATE>` Pop the state off the top of the stack.
+    + `<STATE>` Replace the state stack with the state `STATE`
+5. Token Name, for example `"token"` any non-space character between double quotes, including double quotes.
+6. End of line, finishes each rule.
+
+## User Subroutines
+
+Since lrlex doesn't support user actions, it doesn't support User Subroutines either.