Allow NamedArgs to start with underscore

fixes #1869

named_args.go

@@ -14,6 +14,9 @@ import (
 //
 //	conn.Query(ctx, "select * from widgets where foo = @foo and bar = @bar", pgx.NamedArgs{"foo": 1, "bar": 2})
 //	conn.Query(ctx, "select * from widgets where foo = $1 and bar = $2", 1, 2)
+//
+// Named placeholders are case sensitive and must start with a letter or underscore. Subsequent characters can be
+// letters, numbers, or underscores.
 type NamedArgs map[string]any
 
 // RewriteQuery implements the QueryRewriter interface.
@@ -80,7 +83,7 @@ func rawState(l *sqlLexer) stateFn {
 			return doubleQuoteState
 		case '@':
 			nextRune, _ := utf8.DecodeRuneInString(l.src[l.pos:])
-			if isLetter(nextRune) {
+			if isLetter(nextRune) || nextRune == '_' {
 				if l.pos-l.start > 0 {
 					l.parts = append(l.parts, l.src[l.start:l.pos-width])
 				}

named_args_test.go

@@ -38,10 +38,10 @@ func TestNamedArgsRewriteQuery(t *testing.T) {
 			expectedArgs: []any{int32(42), "foo"},
 		},
 		{
-			sql:          "select @Abc::int, @b_4::text",
-			namedArgs:    pgx.NamedArgs{"Abc": int32(42), "b_4": "foo"},
-			expectedSQL:  "select $1::int, $2::text",
-			expectedArgs: []any{int32(42), "foo"},
+			sql:          "select @Abc::int, @b_4::text, @_c::int",
+			namedArgs:    pgx.NamedArgs{"Abc": int32(42), "b_4": "foo", "_c": int32(1)},
+			expectedSQL:  "select $1::int, $2::text, $3::int",
+			expectedArgs: []any{int32(42), "foo", int32(1)},
 		},
 		{
 			sql:          "at end @",
@@ -50,15 +50,15 @@ func TestNamedArgsRewriteQuery(t *testing.T) {
 			expectedArgs: []any{},
 		},
 		{
-			sql:          "ignores without letter after @ foo bar",
+			sql:          "ignores without valid character after @ foo bar",
 			namedArgs:    pgx.NamedArgs{"a": int32(42), "b": "foo"},
-			expectedSQL:  "ignores without letter after @ foo bar",
+			expectedSQL:  "ignores without valid character after @ foo bar",
 			expectedArgs: []any{},
 		},
 		{
-			sql:          "name must start with letter @1 foo bar",
+			sql:          "name cannot start with number @1 foo bar",
 			namedArgs:    pgx.NamedArgs{"a": int32(42), "b": "foo"},
-			expectedSQL:  "name must start with letter @1 foo bar",
+			expectedSQL:  "name cannot start with number @1 foo bar",
 			expectedArgs: []any{},
 		},
 		{