diff --git a/src/patch/mod.rs b/src/patch/mod.rs
index 194f3166..cec4bea3 100644
--- a/src/patch/mod.rs
+++ b/src/patch/mod.rs
@@ -1,7 +1,6 @@
 pub mod patch_cli;
 
 use globset::Glob;
-use regex::Regex;
 use std::fs::File;
 use std::io::{Read, Write};
 use std::path::{Path, PathBuf};
@@ -571,13 +570,9 @@ fn make_cpu(cmod: &Hash) -> Result<CpuBuilder> {
 
 /// Find left and right indices of enumeration token in specification string
 ///
-/// # Panics
-///
-/// Panics if the expression contains no or more than one token.
-///
 /// # Examples
 ///
-/// ```
+/// ```ignore
 /// let cases = [
 ///     ("RELOAD?", (6, 0)),
 ///     ("TMR[1-57]_MUX", (3, 4)),
@@ -586,20 +581,25 @@ fn make_cpu(cmod: &Hash) -> Result<CpuBuilder> {
 ///     ("CSPT[1][7-9],CSPT[2][0-5]", (4, 0)),
 /// ];
 /// for (spec, (li, ri)) in cases {
-///     assert_eq!(patch::spec_ind(spec), (li, ri));
+///     assert_eq!(spec_ind(spec), Some((li, ri)));
 /// }
 /// ```
 ///
-fn spec_ind(spec: &str) -> (usize, usize) {
+fn spec_ind(spec: &str) -> Option<(usize, usize)> {
+    use once_cell::sync::Lazy;
+    use regex::Regex;
     let spec = spec.split(',').next().unwrap_or(spec);
-    let re =
-        Regex::new(r"^\w*((?:[\?*]|\[\d+(?:-\d+)?\]|\[[a-zA-Z]+(?:-[a-zA-Z]+)?\])+)\w*$").unwrap();
-    let caps = re.captures(spec).unwrap();
+    static RE: Lazy<Regex> = Lazy::new(|| {
+        Regex::new(r"^\w*((?:[\?*]|\[\d+(?:-\d+)?\]|\[[a-zA-Z]+(?:-[a-zA-Z]+)?\])+)\w*$").unwrap()
+    });
+    let Some(caps) = RE.captures(spec) else {
+        return None;
+    };
     let spec = caps.get(0).unwrap();
     let token = caps.get(1).unwrap();
     let li = token.start();
     let ri = spec.end() - token.end();
-    (li, ri)
+    Some((li, ri))
 }
 
 fn check_offsets(offsets: &[u32], dim_increment: u32) -> bool {
diff --git a/src/patch/peripheral.rs b/src/patch/peripheral.rs
index c4d70314..3edb9cd5 100644
--- a/src/patch/peripheral.rs
+++ b/src/patch/peripheral.rs
@@ -1135,7 +1135,11 @@ fn collect_in_array(
         return Err(anyhow!("{path}: registers {rspec} not found"));
     }
     registers.sort_by_key(|r| r.address_offset);
-    let (li, ri) = spec_ind(rspec);
+    let Some((li, ri)) = spec_ind(rspec) else {
+        return Err(anyhow!(
+            "`{rspec}` contains no tokens or contains more than one token"
+        ));
+    };
     let dim = registers.len();
     let dim_index = if rmod.contains_key(&"_start_from_zero".to_yaml()) {
         (0..dim).map(|v| v.to_string()).collect::<Vec<_>>()
@@ -1241,10 +1245,18 @@ fn collect_in_cluster(
                 .iter()
                 .map(|r| {
                     let match_rspec = matchsubspec(&r.name, rspec).unwrap();
-                    let (li, ri) = spec_ind(match_rspec);
-                    r.name[li..r.name.len() - ri].to_string()
+                    let Some((li, ri)) = spec_ind(match_rspec) else {
+                        return Err(anyhow!(
+                            "`{match_rspec}` contains no tokens or contains more than one token"
+                        ));
+                    };
+                    Ok(r.name[li..r.name.len() - ri].to_string())
                 })
-                .collect::<Vec<_>>();
+                .collect::<Result<Vec<_>, _>>();
+            let new_dim_index = match new_dim_index {
+                Ok(v) => v,
+                Err(e) => return Err(e),
+            };
             if first {
                 dim = registers.len();
                 dim_index = new_dim_index;
@@ -1316,7 +1328,11 @@ fn collect_in_cluster(
             reg.name = if let Some(name) = rmod.get_str("name")? {
                 name.into()
             } else {
-                let (li, ri) = spec_ind(&rspec);
+                let Some((li, ri)) = spec_ind(&rspec) else {
+                    return Err(anyhow!(
+                        "`{rspec}` contains no tokens or contains more than one token"
+                    ));
+                };
                 format!("{}{}", &rspec[..li], &rspec[rspec.len() - ri..])
             };
             if let Some(desc) = rmod.get_str("description")? {
diff --git a/src/patch/register.rs b/src/patch/register.rs
index fc40abc9..03c7fea3 100644
--- a/src/patch/register.rs
+++ b/src/patch/register.rs
@@ -373,7 +373,11 @@ impl RegisterExt for Register {
                 return Err(anyhow!("{}: fields {fspec} not found", self.name));
             }
             fields.sort_by_key(|f| f.bit_range.offset);
-            let (li, ri) = spec_ind(fspec);
+            let Some((li, ri)) = spec_ind(fspec) else {
+                return Err(anyhow!(
+                    "`{fspec}` contains no tokens or contains more than one token"
+                ));
+            };
             let dim = fields.len();
             let dim_index = if fmod.contains_key(&"_start_from_zero".to_yaml()) {
                 (0..dim).map(|v| v.to_string()).collect::<Vec<_>>()