test(postgresql): add regression tests for fixed bugs (#41)

Add regression test suite to prevent reintroduction of bugs fixed in this PR.

Tests added:
1. function_names_as_identifiers.sql
   - Tests that function names (exp, div, floor, mod, etc.) work as identifiers
   - Validates fix for removing builtin_function_name tokens

2. json_typecast.sql
   - Tests JSON/JSONB typecast syntax (::json, ::jsonb)
   - Validates fix for missing jsontype rule

3. complex_joins.sql
   - Tests complex multi-table JOINs with ORDER BY
   - General regression test for query patterns

All tests are automatically included in 'make test' via recursive directory processing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: h3n4l

postgresql/examples/regression/README.md

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+# Regression Tests
+
+This directory contains regression tests for specific bugs that were fixed in the parser.
+
+## Test Files
+
+### `function_names_as_identifiers.sql`
+**Issue**: After removing `builtin_function_name` tokens, mathematical and string function names should be usable as regular identifiers.
+
+**Tests**:
+- Using function names (exp, div, floor, mod, power, sqrt, log, reverse) as column names
+- Using function names as table names
+- SELECT statements with these identifiers
+
+**Related PR**: #40 - Remove non-standard builtin_function_name tokens
+
+### `json_typecast.sql`
+**Issue**: Missing `jsontype` rule caused `::json` and `::jsonb` typecasts to fail with "mismatched input ';' expecting '%'"
+
+**Tests**:
+- Basic JSON/JSONB typecast syntax
+- JSON with unicode escape sequences
+- JSON with surrogate pairs
+- JSON type in CREATE TABLE
+- JSON typecast with -> operator
+
+**Related PR**: #40 - Add jsontype rule for proper JSON type support
+
+### `complex_joins.sql`
+**Issue**: Complex multi-table JOINs with multiple LEFT JOINs and ORDER BY clauses
+
+**Tests**:
+- Multiple LEFT JOIN statements
+- Column aliases with AS keyword
+- WHERE clause filtering
+- ORDER BY with multiple columns and mixed ASC/DESC
+
+**Related PR**: #40 - General regression test for complex query patterns
+
+## Running Tests
+
+These tests are automatically included when running the full test suite:
+
+```bash
+cd postgresql
+make test
+```
+
+To run only regression tests:
+
+```bash
+go test -run "TestPostgreSQLParser/examples/regression" -v
+```
+
+## Adding New Regression Tests
+
+When fixing a parser bug:
+
+1. Create a new `.sql` file in this directory
+2. Add a comment at the top describing the issue and fix
+3. Include minimal test cases that reproduce the bug
+4. The test will be automatically picked up by the test runner

postgresql/examples/regression/complex_joins.sql

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+-- Regression test: Complex multi-table JOINs with ORDER BY
+-- Tests parser handling of multiple LEFT JOINs, aliases, and ordering
+
+SELECT
+  se.id,
+  se.name,
+  se.description,
+  st.type AS step_type,
+  st.index AS step_index,
+  sr.conditional,
+  sr.index AS sr_index,
+  script.script_content
+FROM test se
+LEFT JOIN test_step st ON se.id = st.sequence_id
+LEFT JOIN test_rel sr ON sr.step_id = sr.script_id
+LEFT JOIN msg_script script ON script.id = sr.script_id
+WHERE se.creator_id = 1
+ORDER BY se.id DESC, sr_index ASC;

postgresql/examples/regression/function_names_as_identifiers.sql

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+-- Regression test: Function names should be usable as identifiers
+-- Issue: After removing builtin_function_name tokens, mathematical function names
+-- should work as regular identifiers (table names, column names, etc.)
+
+-- Mathematical functions as column names
+CREATE TABLE test_math (
+    exp INT,
+    div INT,
+    floor INT,
+    mod INT,
+    power INT,
+    sqrt INT,
+    log INT
+);
+
+-- String function REVERSE as column name
+CREATE TABLE test_string (
+    reverse VARCHAR(100)
+);
+
+-- Function names as table names
+CREATE TABLE exp (id INT);
+CREATE TABLE div (id INT);
+
+-- Select from tables with function names
+SELECT exp, div, floor, mod FROM test_math;
+SELECT reverse FROM test_string;
+SELECT * FROM exp;

postgresql/examples/regression/json_typecast.sql

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+-- Regression test: JSON and JSONB type casting
+-- Issue: Missing jsontype rule caused ::json and ::jsonb typecasts to fail
+-- Fix: Added jsontype rule to simpletypename and consttypename
+
+-- Basic JSON typecast
+SELECT '{"a": 1}'::json;
+SELECT '{"b": 2}'::jsonb;
+
+-- JSON with unicode escapes
+SELECT '"\u0000"'::json;
+SELECT '"\uaBcD"'::json;
+
+-- JSON with surrogate pairs
+SELECT '{ "a":  "\ud83d\ude04\ud83d\udc36" }'::json;
+
+-- JSON in table definition
+CREATE TABLE json_test (
+    data json,
+    data_b jsonb
+);
+
+-- JSON typecast in expressions
+SELECT '{"a": 1}'::json -> 'a';
+SELECT '{"b": 2}'::jsonb -> 'b';