diff --git a/src/pvxs/unittest.h b/src/pvxs/unittest.h
index c6d9097..d34b370 100644
--- a/src/pvxs/unittest.h
+++ b/src/pvxs/unittest.h
@@ -89,6 +89,13 @@ public:
         msg<<v;
         return *this;
     }
+
+    /** Access to underlying std::ostream used to accumulate notes.
+     *  When our operator<< isn't enough.
+     *  @since UNRELEASED
+     */
+    inline
+    std::ostream& stream() { return msg; }
 };
 
 namespace detail {
diff --git a/src/unittest.cpp b/src/unittest.cpp
index 4cbfe5b..dba7b77 100644
--- a/src/unittest.cpp
+++ b/src/unittest.cpp
@@ -167,15 +167,6 @@ testCase& testCase::setPassMatch(const std::string& expr, const std::string& inp
 
 namespace detail {
 
-static
-size_t findNextLine(const std::string& s, size_t pos=0u)
-{
-    size_t next = s.find_first_of('\n', pos);
-    if(next!=std::string::npos)
-        next++;
-    return next;
-}
-
 testCase _testStrTest(unsigned op, const char *sLHS, const char* rlhs, const char *sRHS, const char* rrhs)
 {
     bool eq;
@@ -187,45 +178,7 @@ testCase _testStrTest(unsigned op, const char *sLHS, const char* rlhs, const cha
         eq = strcmp(rlhs, rrhs)==0;
     testCase ret(eq==op);
     ret<<sLHS<<(op ? " == " : " != ")<<sRHS<<"\n";
-
-    std::string lhs(rlhs ? rlhs : "<null>");
-    std::string rhs(rrhs ? rrhs : "<null>");
-
-    size_t posL=0u, posR=0u;
-
-    while(posL<lhs.size() && posR<rhs.size()) {
-        size_t eolL = findNextLine(lhs, posL);
-        size_t eolR = findNextLine(rhs, posR);
-
-        auto L = lhs.substr(posL, eolL-posL);
-        auto R = rhs.substr(posR, eolR-posR);
-
-        if(L==R) {
-            ret<<"  \""<<escape(L)<<"\"\n";
-        } else {
-            ret<<"+ \""<<escape(R)<<"\"\n";
-            ret<<"- \""<<escape(L)<<"\"\n";
-        }
-
-        posL = eolL;
-        posR = eolR;
-    }
-
-    while(posR<rhs.size()) {
-        size_t eol = findNextLine(rhs, posR);
-        auto line = rhs.substr(posR, eol-posR);
-        ret<<"+ \""<<escape(line)<<"\"\n";
-
-        posR = eol;
-    }
-
-    while(posL<lhs.size()) {
-        size_t eol = findNextLine(lhs, posL);
-        auto line = lhs.substr(posL, eol-posL);
-        ret<<"- \""<<escape(line)<<"\"\n";
-
-        posL = eol;
-    }
+    strDiff(ret.stream(), rlhs, rrhs);
 
     return ret;
 }
diff --git a/src/util.cpp b/src/util.cpp
index ebb915f..70eac89 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -742,4 +742,81 @@ int64_t parseTo<int64_t>(const std::string& s) {
     return ret;
 }
 
+static
+std::vector<std::string>
+splitLines(const char *inp)
+{
+    std::vector<std::string> ret;
+    while(*inp) {
+        auto start = inp;
+        // find next EoL or nil
+        for(char c=*inp; c!='\0' && c!='\n' && c!='\r'; c=*++inp) {}
+        // inp points to EoL or nil
+        ret.emplace_back(start, inp); // copy line w/o EoL
+        // skip past one EoL ("\n", "\r\n", or "\n\r")
+        if(inp[0]=='\n' && inp[1]=='\r') inp+=2u;
+        else if(inp[0]=='\r' && inp[1]=='\n') inp+=2u;
+        else if(inp[0]=='\n') inp+=1u;
+    }
+    return ret;
+}
+
+void strDiff(std::ostream& out,
+             const char *lhs,
+             const char *rhs)
+{
+    if(!lhs)
+        lhs = "<null>";
+    if(!rhs)
+        rhs = "<null>";
+    auto l_lines(splitLines(lhs));
+    auto r_lines(splitLines(rhs));
+    size_t L, R;
+
+    for(L=0u, R=0u; L<l_lines.size() && R<r_lines.size();) {
+        // diagonal search out from current positions
+        for(size_t dist=0u; true; dist++) { // iterate out
+            for(size_t C=0u; C<=dist; C++) { // iterate across
+                size_t testL = L+C;
+                size_t testR = R+(dist-C);
+
+                if(testL>=l_lines.size() && testR>=r_lines.size()) {
+                    goto done;
+                }
+
+                if(testL>=l_lines.size() || testR>=r_lines.size()) {
+                    continue;
+                }
+
+                if(l_lines[testL]==r_lines[testR]) {
+                    // found matching line
+
+                    for(; L < testL; L++) {
+                        out<<"- \""<<escape(l_lines[L])<<"\"\n";
+                    }
+                    for(; R < testR; R++) {
+                        out<<"+ \""<<escape(r_lines[R])<<"\"\n";
+                    }
+                    out<<"  \""<<escape(l_lines[testL])<<"\"\n";
+
+                    L = testL+1u;
+                    R = testR+1u;
+                    goto next;
+                }
+            }
+        }
+next:
+        continue; // oh for lack of a "break N;"
+    }
+done:
+    // print trailing
+    for(; L < l_lines.size(); L++) {
+        out<<"- \""<<escape(l_lines[L])<<"\"\n";
+    }
+    for(; R < r_lines.size(); R++) {
+        out<<"+ \""<<escape(r_lines[R])<<"\"\n";
+    }
+    return;
+}
+
 }}
diff --git a/src/utilpvt.h b/src/utilpvt.h
index b17fd50..b2bb4ec 100644
--- a/src/utilpvt.h
+++ b/src/utilpvt.h
@@ -82,6 +82,10 @@ struct SB {
     SB& operator<<(const T& i) { strm<<i; return *this; }
 };
 
+PVXS_API
+void strDiff(std::ostream& out,
+             const char *lhs,
+             const char *rhs);
 
 void threadOnce(epicsThreadOnceId *id, EPICSTHREADFUNC fn, void *arg=nullptr);
 
diff --git a/test/testutil.cpp b/test/testutil.cpp
index f57862d..93336a3 100644
--- a/test/testutil.cpp
+++ b/test/testutil.cpp
@@ -6,6 +6,7 @@
 
 #include <vector>
 #include <ostream>
+#include <sstream>
 
 #include <epicsUnitTest.h>
 #include <testMain.h>
@@ -204,11 +205,73 @@ void testTestEq()
     testStrMatch("[Hh]ello [Ww]orld", "hello world");
 }
 
+void testStrDiff()
+{
+    testShow()<<__func__;
+
+    const auto testD = [](const char *lhs, const char *rhs, const char *expect) -> bool {
+        std::ostringstream strm;
+        strDiff(strm, lhs, rhs);
+        auto actual(strm.str());
+        return testEq(expect, actual).operator bool();
+    };
+
+    testD("", "", "");
+    testD("one", "one", "  \"one\"\n");
+    testD("one\n",
+          "one\n",
+          "  \"one\"\n");
+    testD("one\n",
+          "two\n",
+          "- \"one\"\n"
+          "+ \"two\"\n");
+    testD("one\n"
+          " aaa\n"
+          "two\n",
+          "one\n"
+          " bbb\n"
+          "two\n",
+          "  \"one\"\n"
+          "- \" aaa\"\n"
+          "+ \" bbb\"\n"
+          "  \"two\"\n");
+    testD("one\n"
+          " aaa\n"
+          "two\n",
+          "one\n"
+          "two\n",
+          "  \"one\"\n"
+          "- \" aaa\"\n"
+          "  \"two\"\n");
+    testD("one\n"
+          "two\n",
+          "one\n"
+          " bbb\n"
+          "two\n",
+          "  \"one\"\n"
+          "+ \" bbb\"\n"
+          "  \"two\"\n");
+    testD("one\n"
+          " aaa\n"
+          "two\n"
+          " xxx\n",
+          "one\n"
+          " bbb\n"
+          "two\n"
+          " yyy\n",
+          "  \"one\"\n"
+          "- \" aaa\"\n"
+          "+ \" bbb\"\n"
+          "  \"two\"\n"
+          "- \" xxx\"\n"
+          "+ \" yyy\"\n");
+}
+
 } // namespace
 
 MAIN(testutil)
 {
-    testPlan(25);
+    testPlan(33);
     testTrue(version_abi_check())<<" 0x"<<std::hex<<PVXS_VERSION<<" ~= 0x"<<std::hex<<PVXS_ABI_VERSION;
     testServerGUID();
     testFill();
@@ -216,5 +279,6 @@ MAIN(testutil)
     testSpamMany();
     testAccount();
     testTestEq();
+    testStrDiff();
     return testDone();
 }