json: document schema conversion in GBNF readme, align manual grammar examples & converters (#7841)

* json: fix char pattern in grammar converters

* json: prevent number precision & whitespace runaways in example grammars

* json: add doc to grammar readme

grammars/README.md

@@ -94,6 +94,8 @@ This guide provides a brief overview. Check out the GBNF files in this directory
 ./main -m <model> --grammar-file grammars/some-grammar.gbnf -p 'Some prompt'
 ```
 
+`llama.cpp` can also convert JSON schemas to grammars either ahead of time or at each request, see below.
+
 ## Troubleshooting
 
 Grammars currently have performance gotchas (see https://github.com/ggerganov/llama.cpp/issues/4218).
@@ -103,3 +105,40 @@ Grammars currently have performance gotchas (see https://github.com/ggerganov/ll
 A common pattern is to allow repetitions of a pattern `x` up to N times.
 
 While semantically correct, the syntax `x? x? x?.... x?` (with N repetitions) may result in extremely slow sampling. Instead, you can write `x{0,N}` (or `(x (x (x ... (x)?...)?)?)?` w/ N-deep nesting in earlier llama.cpp versions).
+
+## Using GBNF grammars
+
+You can use GBNF grammars:
+
+- In the [server](../examples/server)'s completion endpoints, passed as the `grammar` body field
+- In the [main](../examples/main) CLI, passed as the `--grammar` & `--grammar-file` flags
+- With the [gbnf-validator](../examples/gbnf-validator) tool, to test them against strings.
+
+## JSON Schemas → GBNF
+
+`llama.cpp` supports converting a subset of https://json-schema.org/ to GBNF grammars:
+
+- In the [server](../examples/server):
+    - For any completion endpoints, passed as the `json_schema` body field
+    - For the `/chat/completions` endpoint, passed inside the `result_format` body field (e.g. `{"type", "json_object", "schema": {"items": {}}}`)
+- In the [main](../examples/main) CLI, passed as the `--json` / `-j` flag
+- To convert to a grammar ahead of time:
+    - in CLI, with [json_schema_to_grammar.py](../examples/json_schema_to_grammar.py)
+    - in JavaScript with [json-schema-to-grammar.mjs](../examples/server/public/json-schema-to-grammar.mjs) (this is used by the [server](../examples/server)'s Web UI)
+
+Take a look at [tests](../../tests/test-json-schema-to-grammar.cpp) to see which features are likely supported (you'll also find usage examples in https://github.com/ggerganov/llama.cpp/pull/5978, https://github.com/ggerganov/llama.cpp/pull/6659 & https://github.com/ggerganov/llama.cpp/pull/6555).
+
+Here is also a non-exhaustive list of **unsupported** features:
+
+- `additionalProperties`: to be fixed in https://github.com/ggerganov/llama.cpp/pull/7840
+- `minimum`, `exclusiveMinimum`, `maximum`, `exclusiveMaximum`
+    - `integer` constraints to be implemented in https://github.com/ggerganov/llama.cpp/pull/7797
+- Remote `$ref`s in the C++ version (Python & JavaScript versions fetch https refs)
+- Mixing `properties` w/ `anyOf` / `oneOf` in the same type (https://github.com/ggerganov/llama.cpp/issues/7703)
+- `string` formats `uri`, `email`
+- [`contains`](https://json-schema.org/draft/2020-12/json-schema-core#name-contains) / `minContains`
+- `uniqueItems`
+- `$anchor` (cf. [dereferencing](https://json-schema.org/draft/2020-12/json-schema-core#name-dereferencing))
+- [`not`](https://json-schema.org/draft/2020-12/json-schema-core#name-not)
+- [Conditionals](https://json-schema.org/draft/2020-12/json-schema-core#name-keywords-for-applying-subsche) `if` / `then` / `else` / `dependentSchemas`
+- [`patternProperties`](https://json-schema.org/draft/2020-12/json-schema-core#name-patternproperties)

grammars/json.gbnf

@@ -16,10 +16,10 @@ array  ::=
 string ::=
   "\"" (
     [^"\\\x7F\x00-\x1F] |
-    "\\" (["\\/bfnrt] | "u" [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F]) # escapes
+    "\\" (["\\bfnrt] | "u" [0-9a-fA-F]{4}) # escapes
   )* "\"" ws
 
-number ::= ("-"? ([0-9] | [1-9] [0-9]*)) ("." [0-9]+)? ([eE] [-+]? [0-9]+)? ws
+number ::= ("-"? ([0-9] | [1-9] [0-9]{0,15})) ("." [0-9]+)? ([eE] [-+]? [0-9] [1-9]{0,15})? ws
 
 # Optional space: by convention, applied in this grammar after literal chars when allowed
-ws ::= ([ \t\n] ws)?
+ws ::= [ \t\n]{0,20}

grammars/json_arr.gbnf

@@ -25,10 +25,10 @@ array  ::=
 string ::=
   "\"" (
     [^"\\\x7F\x00-\x1F] |
-    "\\" (["\\/bfnrt] | "u" [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F]) # escapes
+    "\\" (["\\bfnrt] | "u" [0-9a-fA-F]{4}) # escapes
   )* "\"" ws
 
-number ::= ("-"? ([0-9] | [1-9] [0-9]*)) ("." [0-9]+)? ([eE] [-+]? [0-9]+)? ws
+number ::= ("-"? ([0-9] | [1-9] [0-9]{0,15})) ("." [0-9]+)? ([eE] [-+]? [1-9] [0-9]{0,15})? ws
 
 # Optional space: by convention, applied in this grammar after literal chars when allowed
-ws ::= ([ \t\n] ws)?
+ws ::= [ \t\n]{0,20}