diff --git a/lexer/src/states/hash.rs b/lexer/src/states/hash.rs
index 9123773..ffccbb5 100644
--- a/lexer/src/states/hash.rs
+++ b/lexer/src/states/hash.rs
@@ -6,7 +6,7 @@ use chars::Lexable;
 use error::Error;
 use states::{State, StateResult};
 use states::bool::Bool;
-use states::number::Prefix;
+use states::number::{Builder, Prefix};
 use token::Token;
 
 trait HashLexable {
@@ -28,7 +28,7 @@ impl State for Hash {
                 StateResult::advance(Box::new(Bool::new(buf.as_str())))
             },
             c if c.is_radix() || c.is_exactness() => {
-                if let Some(st) = Prefix::with_char(c) {
+                if let Some(st) = Prefix::with_char(Builder::new(), c) {
                     StateResult::advance(Box::new(st))
                 } else {
                     StateResult::fail(Error::new(format!("invalid numeric prefix character: {}", c)))
diff --git a/lexer/src/states/number/prefix.rs b/lexer/src/states/number/prefix.rs
index 0bf8a39..dddadfa 100644
--- a/lexer/src/states/number/prefix.rs
+++ b/lexer/src/states/number/prefix.rs
@@ -17,16 +17,24 @@ impl Prefix {
         Prefix(b)
     }
 
-    pub fn with_char(c: char) -> Option<Prefix> {
-        let mut builder = Builder::new();
+    pub fn with_char(b: Builder, c: char) -> Option<Prefix> {
         if let Some(ex) = Exact::from(c) {
-            builder.push_exact(ex);
+            if b.seen_exact() {
+                return None;
+            }
+            let mut b = b.clone();
+            b.push_exact(ex);
+            Some(Prefix::new(b))
         } else if let Some(rx) = Radix::from(c) {
-            builder.push_radix(rx);
+            if b.seen_radix() {
+                return None;
+            }
+            let mut b = b.clone();
+            b.push_radix(rx);
+            Some(Prefix::new(b))
         } else {
-            return None;
+            None
         }
-        Some(Prefix::new(builder))
     }
 }
 
@@ -48,20 +56,8 @@ impl State for Prefix {
 
 impl State for Hash {
     fn lex(&mut self, c: char) -> StateResult {
-        if let Some(ex) = Exact::from(c) {
-            if !self.0.seen_exact() {
-                self.0.push_exact(ex);
-                StateResult::advance(Box::new(Prefix::new(self.0)))
-            } else {
-                StateResult::fail(Error::invalid_char(c))
-            }
-        } else if let Some(rx) = Radix::from(c) {
-            if !self.0.seen_radix() {
-                self.0.push_radix(rx);
-                StateResult::advance(Box::new(Prefix::new(self.0)))
-            } else {
-                StateResult::fail(Error::invalid_char(c))
-            }
+        if let Some(st) = Prefix::with_char(self.0, c) {
+            StateResult::advance(Box::new(st))
         } else {
             StateResult::fail(Error::invalid_char(c))
         }