diff --git a/src/util.cc b/src/util.cc
index bfa292a8..857a848e 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -66,16 +66,6 @@ const char DEFAULT_STRIP_CHARSET[] = "\r\n\t ";
 
 const char UPPER_XDIGITS[] = "0123456789ABCDEF";
 
-bool isAlpha(const char c) {
-  return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z');
-}
-
-bool isDigit(const char c) { return '0' <= c && c <= '9'; }
-
-bool isHexDigit(const char c) {
-  return isDigit(c) || ('A' <= c && c <= 'F') || ('a' <= c && c <= 'f');
-}
-
 bool inRFC3986UnreservedChars(const char c) {
   static const char unreserved[] = {'-', '.', '_', '~'};
   return isAlpha(c) || isDigit(c) ||
diff --git a/src/util.h b/src/util.h
index 6ca9f735..7524a247 100644
--- a/src/util.h
+++ b/src/util.h
@@ -158,11 +158,15 @@ std::string joinPath(InputIterator first, InputIterator last) {
   return strjoin(elements.begin(), elements.end(), "/");
 }
 
-bool isAlpha(const char c);
+inline bool isAlpha(const char c) {
+  return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z');
+}
 
-bool isDigit(const char c);
+inline bool isDigit(const char c) { return '0' <= c && c <= '9'; }
 
-bool isHexDigit(const char c);
+inline bool isHexDigit(const char c) {
+  return isDigit(c) || ('A' <= c && c <= 'F') || ('a' <= c && c <= 'f');
+}
 
 bool inRFC3986UnreservedChars(const char c);