diff --git a/src/proc_macros/Cargo.toml b/src/proc_macros/Cargo.toml
index b94737d..e13b48c 100644
--- a/src/proc_macros/Cargo.toml
+++ b/src/proc_macros/Cargo.toml
@@ -7,7 +7,7 @@ license = "MIT"
 edition = "2018"
 
 [dependencies]
-syn = { version = "1.0", features = ["full"] }
+syn = { version = "1.0.103", features = ["full"] }
 proc-macro2 = "1.0"
 quote = "1.0"
 regex = "1.5"
diff --git a/src/proc_macros/regex_code.rs b/src/proc_macros/regex_code.rs
index 06bdc31..7658656 100644
--- a/src/proc_macros/regex_code.rs
+++ b/src/proc_macros/regex_code.rs
@@ -29,7 +29,7 @@ impl TryFrom<LitStr> for RegexCode {
         let mut ignore_whitespace = false;
         let mut swap_greed = false;
         let mut is_bytes = false;
-        for ch in lit_str.suffix().chars() {
+        for (i, ch) in lit_str.suffix().chars().enumerate() {
             match ch {
                 'i' => case_insensitive = true,
                 'm' => multi_line = true,
@@ -38,7 +38,13 @@ impl TryFrom<LitStr> for RegexCode {
                 'U' => swap_greed = true,
                 'B' => is_bytes = true, // non-standard!
                 _ => {
-                    panic!("unrecognized regex flag {:?}", ch);
+                    let lit = lit_str.token();
+                    let pos = lit.to_string().len() - i;
+                    // subspan only works on nighlty
+                    return Err(syn::Error::new(
+                        lit.subspan(pos - 1..pos).unwrap_or_else(|| lit.span()),
+                        format!("unrecognized regex flag {:?}", ch),
+                    ));
                 }
             };
         }