[pyupgrade] Fix UP030 to avoid modifying double curly braces in format strings (#19378)

## Summary

Fixes #19348

---------

Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>

crates/ruff_linter/src/rules/pyupgrade/rules/format_literals.rs

@@ -124,10 +124,20 @@ fn is_sequential(indices: &[usize]) -> bool {
     indices.iter().enumerate().all(|(idx, value)| idx == *value)
 }
 
-// An opening curly brace, followed by any integer, followed by any text,
-// followed by a closing brace.
-static FORMAT_SPECIFIER: LazyLock<Regex> =
-    LazyLock::new(|| Regex::new(r"\{(?P<int>\d+)(?P<fmt>.*?)}").unwrap());
+static FORMAT_SPECIFIER: LazyLock<Regex> = LazyLock::new(|| {
+    Regex::new(
+        r"(?x)
+            (?P<prefix>
+                ^|[^{]|(?:\{{2})+  # preceded by nothing, a non-brace, or an even number of braces
+            )
+            \{                     # opening curly brace
+                (?P<int>\d+)       # followed by any integer
+                (?P<fmt>.*?)       # followed by any text
+            }                      # followed by a closing brace
+        ",
+    )
+    .unwrap()
+});
 
 /// Remove the explicit positional indices from a format string.
 fn remove_specifiers<'a>(value: &mut Expression<'a>, arena: &'a typed_arena::Arena<String>) {
@@ -135,7 +145,7 @@ fn remove_specifiers<'a>(value: &mut Expression<'a>, arena: &'a typed_arena::Are
         Expression::SimpleString(expr) => {
             expr.value = arena.alloc(
                 FORMAT_SPECIFIER
-                    .replace_all(expr.value, "{$fmt}")
+                    .replace_all(expr.value, "$prefix{$fmt}")
                     .to_string(),
             );
         }
@@ -146,7 +156,7 @@ fn remove_specifiers<'a>(value: &mut Expression<'a>, arena: &'a typed_arena::Are
                     libcst_native::String::Simple(string) => {
                         string.value = arena.alloc(
                             FORMAT_SPECIFIER
-                                .replace_all(string.value, "{$fmt}")
+                                .replace_all(string.value, "$prefix{$fmt}")
                                 .to_string(),
                         );
                     }

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP030_0.py.snap

@@ -481,6 +481,7 @@ UP030_0.py:59:1: UP030 [*] Use implicit references for positional format fields
    59 |+"{}_{}".format(2, 1, )
 60 60 | 
 61 61 | "{1}_{0}".format(1, 2)
+62 62 | 
 
 UP030_0.py:61:1: UP030 [*] Use implicit references for positional format fields
    |
@@ -488,6 +489,8 @@ UP030_0.py:61:1: UP030 [*] Use implicit references for positional format fields
 60 |
 61 | "{1}_{0}".format(1, 2)
    | ^^^^^^^^^^^^^^^^^^^^^^ UP030
+62 |
+63 | r"\d{{1,2}} {0}".format(42)
    |
    = help: Remove explicit positional indices
 
@@ -497,3 +500,42 @@ UP030_0.py:61:1: UP030 [*] Use implicit references for positional format fields
 60 60 | 
 61    |-"{1}_{0}".format(1, 2)
    61 |+"{}_{}".format(2, 1)
+62 62 | 
+63 63 | r"\d{{1,2}} {0}".format(42)
+64 64 | 
+
+UP030_0.py:63:1: UP030 [*] Use implicit references for positional format fields
+   |
+61 | "{1}_{0}".format(1, 2)
+62 |
+63 | r"\d{{1,2}} {0}".format(42)
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ UP030
+64 |
+65 | "{{{0}}}".format(123)
+   |
+   = help: Remove explicit positional indices
+
+ℹ Unsafe fix
+60 60 | 
+61 61 | "{1}_{0}".format(1, 2)
+62 62 | 
+63    |-r"\d{{1,2}} {0}".format(42)
+   63 |+r"\d{{1,2}} {}".format(42)
+64 64 | 
+65 65 | "{{{0}}}".format(123)
+
+UP030_0.py:65:1: UP030 [*] Use implicit references for positional format fields
+   |
+63 | r"\d{{1,2}} {0}".format(42)
+64 |
+65 | "{{{0}}}".format(123)
+   | ^^^^^^^^^^^^^^^^^^^^^ UP030
+   |
+   = help: Remove explicit positional indices
+
+ℹ Unsafe fix
+62 62 | 
+63 63 | r"\d{{1,2}} {0}".format(42)
+64 64 | 
+65    |-"{{{0}}}".format(123)
+   65 |+"{{{}}}".format(123)