diff --git a/ChangeLog b/ChangeLog
index 8cf93186..a194440a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2022-05-15  Jay Berkenbilt  <ejb@ql.org>
+
+        * Add QUtil::is_long_long to test whether a string can be
+        converted to a long long and back without loss of information.
+
 2022-05-04  Jay Berkenbilt  <ejb@ql.org>
 
         * JSON: add a new "blob" type that takes a function to write data
diff --git a/include/qpdf/QUtil.hh b/include/qpdf/QUtil.hh
index 8b2b5ff8..5d33b2a5 100644
--- a/include/qpdf/QUtil.hh
+++ b/include/qpdf/QUtil.hh
@@ -65,6 +65,13 @@ namespace QUtil
     QPDF_DLL
     unsigned int string_to_uint(char const* str);
 
+    // Returns true if this exactly represents a long long. The
+    // determination is made by converting the string to a long long,
+    // then converting the result back to a string, and then comparing
+    // that result with the original string.
+    QPDF_DLL
+    bool is_long_long(char const* str);
+
     // Pipeline's write method wants unsigned char*, but we often have
     // some other type of string.  These methods do combinations of
     // const_cast and reinterpret_cast to give us an unsigned char*.
diff --git a/libqpdf/QUtil.cc b/libqpdf/QUtil.cc
index 22962199..8edfadae 100644
--- a/libqpdf/QUtil.cc
+++ b/libqpdf/QUtil.cc
@@ -467,6 +467,19 @@ QUtil::string_to_uint(char const* str)
     return QIntC::to_uint(string_to_ull(str));
 }
 
+bool
+QUtil::is_long_long(char const* str)
+{
+    try {
+        auto i1 = string_to_ll(str);
+        std::string s1 = int_to_string(i1);
+        return str == s1;
+    } catch (std::exception&) {
+        // overflow or other error
+    }
+    return false;
+}
+
 unsigned char*
 QUtil::unsigned_char_pointer(std::string const& str)
 {
diff --git a/libtests/qtest/qutil/qutil.out b/libtests/qtest/qutil/qutil.out
index e9c53170..d91acba9 100644
--- a/libtests/qtest/qutil/qutil.out
+++ b/libtests/qtest/qutil/qutil.out
@@ -129,3 +129,5 @@ delete file
 D:20210209144925-05'00'
 D:20210210011925+05'30'
 D:20210209191925Z
+---- is_long_long
+done
diff --git a/libtests/qutil.cc b/libtests/qutil.cc
index f79f9a3f..ea0aece7 100644
--- a/libtests/qutil.cc
+++ b/libtests/qutil.cc
@@ -676,6 +676,29 @@ timestamp_test()
         QUtil::qpdf_time_to_pdf_time(QUtil::get_current_qpdf_time())));
 }
 
+void
+is_long_long_test()
+{
+    auto check = [](char const* s, bool v) {
+        if (QUtil::is_long_long(s) != v) {
+            std::cout << "failed: " << s << std::endl;
+        }
+    };
+    check("12312312", true);
+    check("12312312.34", false);
+    check("-12312312", true);
+    check("-12312312.34", false);
+    check("1e2", false);
+    check("9223372036854775807", true);
+    check("9223372036854775808", false);
+    check("-9223372036854775808", true);
+    check("-9223372036854775809", false);
+    check("123123123123123123123123123123123123", false);
+    check("potato", false);
+    check("0123", false);
+    std::cout << "done" << std::endl;
+}
+
 int
 main(int argc, char* argv[])
 {
@@ -710,6 +733,8 @@ main(int argc, char* argv[])
         rename_delete_test();
         std::cout << "---- timestamp" << std::endl;
         timestamp_test();
+        std::cout << "---- is_long_long" << std::endl;
+        is_long_long_test();
     } catch (std::exception& e) {
         std::cout << "unexpected exception: " << e.what() << std::endl;
     }