diff --git a/applications/test/fileOperation1/Make/files b/applications/test/fileOperation1/Make/files
new file mode 100644
index 0000000000..3f4a231f16
--- /dev/null
+++ b/applications/test/fileOperation1/Make/files
@@ -0,0 +1,3 @@
+Test-fileOperation1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-fileOperation1
diff --git a/applications/test/fileOperation1/Make/options b/applications/test/fileOperation1/Make/options
new file mode 100644
index 0000000000..18e6fe47af
--- /dev/null
+++ b/applications/test/fileOperation1/Make/options
@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */
diff --git a/applications/test/fileOperation1/Test-fileOperation1.C b/applications/test/fileOperation1/Test-fileOperation1.C
new file mode 100644
index 0000000000..be4480d12e
--- /dev/null
+++ b/applications/test/fileOperation1/Test-fileOperation1.C
@@ -0,0 +1,125 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2020 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-fileOperation1
+
+Description
+   Test string parsing and other bits for fileOperation
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "fileName.H"
+#include "fileOperation.H"
+#include "SubList.H"
+#include "IOobject.H"
+#include "IOstreams.H"
+#include "OSspecific.H"
+
+
+using namespace Foam;
+
+word toString(const fileOperation::procRangeType& group)
+{
+    if (group.empty())
+    {
+        return word::null;
+    }
+    return Foam::name(group.first()) + "-" + Foam::name(group.last());
+}
+
+
+void testSplitPath(const fileName& pathName)
+{
+    fileName path, procDir, local;
+    fileOperation::procRangeType group;
+    label nProcs;
+
+    const label proci =
+        fileOperation::splitProcessorPath
+        (
+            pathName,
+            path,
+            procDir,
+            local,
+            group,
+            nProcs
+        );
+
+
+    Info<< nl
+        << "Input    = " << pathName << nl
+        << "  path   = " << path << nl
+        << "  proc   = " << procDir << nl
+        << "  local  = " << local << nl
+        << "  group  = " << group << " = " << toString(group) << nl
+        << "  proci   = " << proci << nl
+        << "  nProcs  = " << nProcs << nl;
+}
+
+
+void testSplitPaths(std::initializer_list<const char* const> dirNames)
+{
+    for (const auto& dirName : dirNames)
+    {
+        testSplitPath(fileName(dirName));
+    }
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    argList::addArgument("fileName .. fileNameN");
+    argList::addOption("istream", "file", "test Istream values");
+
+
+    testSplitPaths
+    ({
+        "foo/bar",
+        "foo/processor5/system",
+        "foo/processors100_0-5/constant",
+        "foo/processors20_12-16/constant",
+        "/new-processor-gen/case1/processors20",
+        "/new-processor-gen/case1/processors100_0-5/constant",
+        "/new-processor-gen/case1/processors/input",
+        "devel/processor/ideas/processor0/system",
+
+        "/path/processor0Generation1/case1/processor10/input",
+
+        "path/processors100_ab-cd/constant",
+        "path/processors100_a11-d00/constant",
+    });
+
+
+    Info<< "\nEnd\n" << endl;
+    return 0;
+}
+
+
+// ************************************************************************* //
diff --git a/applications/utilities/miscellaneous/foamListTimes/foamListTimes.C b/applications/utilities/miscellaneous/foamListTimes/foamListTimes.C
index d7469d9f05..82d78c654d 100644
--- a/applications/utilities/miscellaneous/foamListTimes/foamListTimes.C
+++ b/applications/utilities/miscellaneous/foamListTimes/foamListTimes.C
@@ -66,19 +66,14 @@ using namespace Foam;
 // Many ways to name processor directories
 //
 // Uncollated       | "processor0", "processor1" ...
-// Collated (old)   | "processors"
-// Collated (new)   | "processors<N>"
+// Collated         | "processors<N>"
 // Host collated    | "processors<N>_<low>-<high>"
 
 const regExp matcher("processors?[0-9]+(_[0-9]+-[0-9]+)?");
 
 bool isProcessorDir(const string& dir)
 {
-    return
-    (
-        dir.starts_with("processor")
-     && (dir == "processors" || matcher.match(dir))
-    );
+    return (dir.starts_with("processor") && matcher.match(dir));
 }
 
 
diff --git a/applications/utilities/miscellaneous/foamRestoreFields/foamRestoreFields.C b/applications/utilities/miscellaneous/foamRestoreFields/foamRestoreFields.C
index 45235ea0d5..03518dcc0c 100644
--- a/applications/utilities/miscellaneous/foamRestoreFields/foamRestoreFields.C
+++ b/applications/utilities/miscellaneous/foamRestoreFields/foamRestoreFields.C
@@ -71,19 +71,14 @@ using namespace Foam;
 // Many ways to name processor directories
 //
 // Uncollated       | "processor0", "processor1" ...
-// Collated (old)   | "processors"
-// Collated (new)   | "processors<N>"
+// Collated         | "processors<N>"
 // Host collated    | "processors<N>_<low>-<high>"
 
 const regExp matcher("processors?[0-9]+(_[0-9]+-[0-9]+)?");
 
 bool isProcessorDir(const string& dir)
 {
-    return
-    (
-        dir.starts_with("processor")
-     && (dir == "processors" || matcher.match(dir))
-    );
+    return (dir.starts_with("processor") && matcher.match(dir));
 }
 
 
@@ -384,13 +379,7 @@ int main(int argc, char *argv[])
         {
             if (leadProcIdx < 0)
             {
-                // Collated (old)
-                leadProcIdx = procDirs.find("processors");
-            }
-
-            if (leadProcIdx < 0)
-            {
-                // Collated (new)
+                // Collated
                 leadProcIdx = procDirs.find("processors" + Foam::name(nProcs));
             }
 
diff --git a/applications/utilities/parallelProcessing/decomposePar/decomposePar.C b/applications/utilities/parallelProcessing/decomposePar/decomposePar.C
index 0fef72da77..bbbc6b63bf 100644
--- a/applications/utilities/parallelProcessing/decomposePar/decomposePar.C
+++ b/applications/utilities/parallelProcessing/decomposePar/decomposePar.C
@@ -519,29 +519,26 @@ int main(int argc, char *argv[])
                 {
                     const fileName& d = dirs[diri];
 
-                    // Starts with 'processors'
-                    if (d.find("processors") == 0)
+                    label proci = -1;
+
+                    if
+                    (
+                        d.starts_with("processor")
+                     &&
+                        (
+                            // Collated is "processors"
+                            d[9] == 's'
+
+                            // Uncollated has integer(s) after 'processor'
+                         || Foam::read(d.substr(9), proci)
+                        )
+                    )
                     {
                         if (fileHandler().exists(d))
                         {
                             fileHandler().rmDir(d);
                         }
                     }
-
-                    // Starts with 'processor'
-                    if (d.find("processor") == 0)
-                    {
-                        // Check that integer after processor
-                        fileName num(d.substr(9));
-                        label proci = -1;
-                        if (Foam::read(num.c_str(), proci))
-                        {
-                            if (fileHandler().exists(d))
-                            {
-                                fileHandler().rmDir(d);
-                            }
-                        }
-                    }
                 }
 
                 procDirsProblem = false;
diff --git a/applications/utilities/postProcessing/miscellaneous/profilingSummary/profilingSummary.C b/applications/utilities/postProcessing/miscellaneous/profilingSummary/profilingSummary.C
index a9fe33dc08..f37f7bc685 100644
--- a/applications/utilities/postProcessing/miscellaneous/profilingSummary/profilingSummary.C
+++ b/applications/utilities/postProcessing/miscellaneous/profilingSummary/profilingSummary.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2017 OpenCFD Ltd.
+    Copyright (C) 2017-2020 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -86,15 +86,7 @@ int main(int argc, char *argv[])
     #include "createTime.H"
 
     // Determine the processor count
-    #ifdef fileOperation_H
     const label nProcs = fileHandler().nProcs(args.path());
-    #else
-    label nProcs = 0;
-    while (isDir(args.path()/("processor" + Foam::name(nProcs))))
-    {
-        ++nProcs;
-    }
-    #endif
 
     // Create the processor databases
     PtrList<Time> databases(nProcs);
diff --git a/src/OpenFOAM/global/fileOperations/collatedFileOperation/OFstreamCollator.C b/src/OpenFOAM/global/fileOperations/collatedFileOperation/OFstreamCollator.C
index a8a1a93960..8fa91a0baa 100644
--- a/src/OpenFOAM/global/fileOperations/collatedFileOperation/OFstreamCollator.C
+++ b/src/OpenFOAM/global/fileOperations/collatedFileOperation/OFstreamCollator.C
@@ -151,9 +151,9 @@ bool Foam::OFstreamCollator::writeFile
         if (UPstream::master(comm))
         {
             off_t sum = 0;
-            forAll(recvSizes, i)
+            for (const label recv : recvSizes)
             {
-                sum += recvSizes[i];
+                sum += recv;
             }
             // Use ostringstream to display long int (until writing these is
             // supported)
diff --git a/src/OpenFOAM/global/fileOperations/collatedFileOperation/collatedFileOperation.C b/src/OpenFOAM/global/fileOperations/collatedFileOperation/collatedFileOperation.C
index 935084e476..a984a57b38 100644
--- a/src/OpenFOAM/global/fileOperations/collatedFileOperation/collatedFileOperation.C
+++ b/src/OpenFOAM/global/fileOperations/collatedFileOperation/collatedFileOperation.C
@@ -142,24 +142,18 @@ bool Foam::fileOperations::collatedFileOperation::appendObject
 
     const bool isMaster = isMasterRank(proci);
 
-    // Determine the local rank if the pathName is a per-rank one
+    // Determine local rank (offset) if the pathName is a per-rank one
     label localProci = proci;
     {
         fileName path, procDir, local;
-        label groupStart, groupSize, nProcs;
-        splitProcessorPath
-        (
-            pathName,
-            path,
-            procDir,
-            local,
-            groupStart,
-            groupSize,
-            nProcs
-        );
-        if (groupSize > 0 && groupStart != -1)
+        procRangeType group;
+        label nProcs;
+        splitProcessorPath(pathName, path, procDir, local, group, nProcs);
+
+        // The local rank (offset)
+        if (!group.empty())
         {
-            localProci = proci-groupStart;
+            localProci = proci - group.start();
         }
     }
 
diff --git a/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.C b/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.C
index 8071a10c4d..4ee216eb7b 100644
--- a/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.C
+++ b/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.C
@@ -36,6 +36,8 @@ License
 #include "polyMesh.H"
 #include "registerSwitch.H"
 #include "Time.H"
+#include <cerrno>
+#include <cinttypes>
 
 /* * * * * * * * * * * * * * * Static Member Data  * * * * * * * * * * * * * */
 
@@ -56,11 +58,6 @@ namespace Foam
     );
 }
 
-
-Foam::autoPtr<Foam::fileOperation> Foam::fileOperation::fileHandlerPtr_;
-
-Foam::word Foam::fileOperation::processorsBaseDir = "processors";
-
 const Foam::Enum<Foam::fileOperation::pathType>
 Foam::fileOperation::pathTypeNames_
 ({
@@ -79,6 +76,109 @@ Foam::fileOperation::pathTypeNames_
 });
 
 
+Foam::word Foam::fileOperation::processorsBaseDir = "processors";
+
+Foam::autoPtr<Foam::fileOperation> Foam::fileOperation::fileHandlerPtr_;
+
+
+// * * * * * * * * * * * * * * * Local Functions * * * * * * * * * * * * * * //
+
+namespace
+{
+
+// Need to parse the numbers
+// from "processors(\d+)" and
+// from "processors(\d+)_(\d+)-(\d+)"
+//
+// Receive the string matching "^(\d+)(?:_(\d+)-(\d+))?/?$"
+//
+//    \1 = numProcs
+//    \2 = firstProc
+//    \3 = lastProc
+//
+// Return true on success and set parameters numProcs and group (size,start)
+//
+// Use low-level C-string to integer parsing to drive the sequence.
+//
+// For simplicity, also skip INT_MAX checks everywhere but check for
+// - (errno) for success
+// - (nptr == endptr) for leading junk
+// - (*endptr != endChar) for trailing junk
+// - skip INT_MAX checks as being too pessimistic
+
+static bool parseProcsNumRange
+(
+    const std::string str,
+    int& numProcs,
+    Foam::fileOperation::procRangeType& group
+)
+{
+    const char * nptr = str.c_str();
+    char *endptr = nullptr;
+
+    // 1. numProcs
+    errno = 0;
+    intmax_t parsed = std::strtoimax(nptr, &endptr, 10);
+    if (errno || nptr == endptr) return false;  // bad parse
+
+    const int nProcs = int(parsed);
+
+    // End of string? Then no range and we are done.
+    if (*endptr == '\0')
+    {
+        numProcs = nProcs;
+        return true;
+    }
+
+    // Parse point at start of range ('_' character)?
+    if (*endptr != '_') return false;
+    nptr = ++endptr;
+
+
+    // 2. firstProc
+    errno = 0;
+    parsed = std::strtoimax(nptr, &endptr, 10);
+    if (errno || nptr == endptr) return false;  // bad parse
+
+    const int firstProc = int(parsed);
+
+    // Parse point at range separator ('-' character)?
+    if (*endptr != '-') return false;
+    nptr = ++endptr;
+
+
+    // 3. lastProc
+    errno = 0;
+    parsed = std::strtoimax(nptr, &endptr, 10);
+    if (errno || nptr == endptr) return false;  // bad parse
+
+    const int lastProc = int(parsed);
+
+
+    if
+    (
+        // Parse point at end of string
+        (*endptr == '\0')
+
+        // Input plausibility
+        // Accept nProcs == 0 in case that becomes useful in the future
+     && (nProcs >= 0 && firstProc >= 0 && firstProc <= lastProc)
+    )
+    {
+        numProcs = nProcs;
+
+        // Convert first/last to start/size
+        group.reset(firstProc, lastProc-firstProc+1);
+
+        return true;
+    }
+
+    return false;
+}
+
+} // End anonymous namespace
+
+
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
 Foam::fileMonitor& Foam::fileOperation::monitor() const
@@ -100,17 +200,17 @@ Foam::fileMonitor& Foam::fileOperation::monitor() const
 
 Foam::instantList Foam::fileOperation::sortTimes
 (
-    const fileNameList& dirEntries,
+    const fileNameList& dirNames,
     const word& constantName
 )
 {
     // Initialise instant list
-    instantList times(dirEntries.size() + 1);
+    instantList times(dirNames.size() + 1);
     label nTimes = 0;
 
     // Check for "constant"
     bool haveConstant = false;
-    for (const fileName& dirName : dirEntries)
+    for (const fileName& dirName : dirNames)
     {
         if (dirName == constantName)
         {
@@ -123,7 +223,7 @@ Foam::instantList Foam::fileOperation::sortTimes
     }
 
     // Read and parse all the entries in the directory
-    for (const fileName& dirName : dirEntries)
+    for (const fileName& dirName : dirNames)
     {
         scalar timeValue;
         if (readScalar(dirName, timeValue))
@@ -244,14 +344,11 @@ Foam::fileOperation::lookupAndCacheProcessorsPath
     // find the corresponding actual processor directory (e.g. 'processors4')
     // and index (2)
 
-    fileName path;
-    fileName pDir;
-    fileName local;
-    label gStart;
-    label gSz;
+    fileName path, pDir, local;
+    procRangeType group;
     label numProcs;
-    label proci =
-        splitProcessorPath(fName, path, pDir, local, gStart, gSz, numProcs);
+    const label proci =
+        splitProcessorPath(fName, path, pDir, local, group, numProcs);
 
     if (proci != -1)
     {
@@ -278,15 +375,14 @@ Foam::fileOperation::lookupAndCacheProcessorsPath
         // - directory+offset containing data for proci
         label maxProc = -1;
 
-        forAll(dirNames, i)
+        for (const fileName& dirN : dirNames)
         {
-            const fileName& dirN = dirNames[i];
-
             // Analyse directory name
             fileName rp, rd, rl;
-            label rStart, rSize, rNum;
-            label readProci =
-                splitProcessorPath(dirN, rp, rd, rl, rStart, rSize, rNum);
+            label rNum;
+            const label readProci =
+                splitProcessorPath(dirN, rp, rd, rl, group, rNum);
+
             maxProc = max(maxProc, readProci);
 
             if (proci == readProci)
@@ -301,16 +397,17 @@ Foam::fileOperation::lookupAndCacheProcessorsPath
                     )
                 );
             }
-            else if (proci >= rStart && proci < rStart+rSize)
+            else if (group.found(proci))
             {
                 // "processorsDDD_start-end"
                 // Found the file that contains the data for proci
+                const label localProci = proci - group.start();
                 procDirs.append
                 (
                     dirIndex
                     (
                         dirN,
-                        Tuple2<pathType, label>(PROCOBJECT, proci-rStart)
+                        Tuple2<pathType, label>(PROCOBJECT, localProci)
                     )
                 );
             }
@@ -319,7 +416,7 @@ Foam::fileOperation::lookupAndCacheProcessorsPath
                 // Direct detection of processorsDDD
                 maxProc = rNum-1;
 
-                if (rStart == -1)
+                if (group.empty())
                 {
                     // "processorsDDD"
                     procDirs.append
@@ -419,13 +516,19 @@ bool Foam::fileOperation::exists(IOobject& io) const
 
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
-Foam::fileOperation::fileOperation(label comm)
+Foam::fileOperation::fileOperation
+(
+    const label comm,
+    const bool distributedRoots
+)
 :
-    comm_(comm)
+    comm_(comm),
+    distributed_(distributedRoots)
 {}
 
 
-Foam::autoPtr<Foam::fileOperation> Foam::fileOperation::New
+Foam::autoPtr<Foam::fileOperation>
+Foam::fileOperation::New
 (
     const word& handlerType,
     bool verbose
@@ -471,7 +574,7 @@ bool Foam::fileOperation::writeObject
 {
     if (valid)
     {
-        fileName pathName(io.objectPath());
+        const fileName pathName(io.objectPath());
 
         mkDir(pathName.path());
 
@@ -482,7 +585,7 @@ bool Foam::fileOperation::writeObject
             return false;
         }
 
-        OSstream& os = osPtr();
+        OSstream& os = *osPtr;
 
         // If any of these fail, return (leave error handling to Ostream class)
         if (!os.good())
@@ -514,14 +617,11 @@ Foam::fileName Foam::fileOperation::filePath(const fileName& fName) const
         Pout<< "fileOperation::filePath :" << " fName:" << fName << endl;
     }
 
-    fileName path;
-    fileName pDir;
-    fileName local;
-    label gStart;
-    label gSz;
+    fileName path, pDir, local;
+    procRangeType group;
     label numProcs;
     label proci =
-        splitProcessorPath(fName, path, pDir, local, gStart, gSz, numProcs);
+        splitProcessorPath(fName, path, pDir, local, group, numProcs);
 
     if (numProcs != -1)
     {
@@ -533,9 +633,9 @@ Foam::fileName Foam::fileOperation::filePath(const fileName& fName) const
     {
         // Get all processor directories
         refPtr<dirIndexList> procDirs(lookupProcessorsPath(fName));
-        forAll(procDirs(), i)
+        for (const dirIndex& dirIdx : procDirs())
         {
-            const fileName& procDir = procDirs()[i].first();
+            const fileName& procDir = dirIdx.first();
 
             fileName collatedName(path/procDir/local);
             if (exists(collatedName))
@@ -690,9 +790,9 @@ Foam::instantList Foam::fileOperation::findTimes
 
     // Get all processor directories
     refPtr<dirIndexList> procDirs(lookupProcessorsPath(directory));
-    forAll(procDirs(), i)
+    for (const dirIndex& dirIdx : procDirs())
     {
-        const fileName& procDir = procDirs()[i].first();
+        const fileName& procDir = dirIdx.first();
         fileName collDir(processorsPath(directory, procDir));
         if (!collDir.empty() && collDir != directory)
         {
@@ -925,21 +1025,20 @@ Foam::label Foam::fileOperation::nProcs
 
         // Detect any processorsDDD or processorDDD
         label maxProc = -1;
-        forAll(dirNames, i)
+        for (const fileName& dirN : dirNames)
         {
-            const fileName& dirN = dirNames[i];
+            fileName rp, rd, rl;
+            procRangeType group;
+            label rNum;
 
-            fileName path, pDir, local;
-            label start, size, n;
-            maxProc = max
-            (
-                maxProc,
-                splitProcessorPath(dirN, path, pDir, local, start, size, n)
-            );
-            if (n != -1)
+            const label readProci =
+                splitProcessorPath(dirN, rp, rd, rl, group, rNum);
+
+            maxProc = max(maxProc, readProci);
+            if (rNum != -1)
             {
                 // Direct detection of processorsDDD
-                maxProc = n-1;
+                maxProc = rNum-1;
                 break;
             }
         }
@@ -1018,12 +1117,13 @@ Foam::fileName Foam::fileOperation::processorsPath
 ) const
 {
     // Check if directory is processorDDD
-    word caseName(dir.name());
 
-    std::string::size_type pos = caseName.find("processor");
-    if (pos == 0)
+    const word caseName(dir.name());
+    if (caseName.starts_with("processor"))
     {
-        if (caseName.size() <= 9 || caseName[9] == 's')
+        // Reject both '^processor$' and '^processors.*$'
+
+        if (!std::isdigit(caseName[9]))
         {
             WarningInFunction << "Directory " << dir
                 << " does not end in old-style processorDDD" << endl;
@@ -1038,131 +1138,172 @@ Foam::fileName Foam::fileOperation::processorsPath
 
 Foam::label Foam::fileOperation::splitProcessorPath
 (
-    const fileName& objectPath,
+    const fileName& objPath,
     fileName& path,
     fileName& procDir,
     fileName& local,
 
-    label& groupStart,
-    label& groupSize,
-
+    procRangeType& group,
     label& nProcs
 )
 {
+    // Return value
+    label returnProci = -1;
+
+    // Clear out the return parameters
+
     path.clear();
     procDir.clear();
     local.clear();
+    group.clear();
 
-    // Potentially detected start of number of processors in local group
-    groupStart = -1;
-    groupSize = 0;
-
-    // Potentially detected number of processors
+    // Invalidate detected number of processors
     nProcs = -1;
 
-    // Search for processor at start of line or /processor
-    std::string::size_type pos = objectPath.find("processor");
-    if (pos == string::npos)
+    // The local processor group is read as first/last, but stored as
+    // start/size.  Empty with start=0, size=0 if no range is detected
+
+
+    // Start of 'processor..' directory name (the procDir)
+    size_t pos = 0;
+
+    // The slash starting the trailing (local) directory
+    size_t slashLocal = string::npos;
+
+
+    // Search for processor at start of string or after /processor
+    //
+    // 'processor(\d+)'
+    // 'processors(\d+)'
+    // 'processors(\d+)_(\d+)-(\d+)'
+
+    for
+    (
+        /*nil*/;
+        (pos = objPath.find("processor", pos)) != string::npos;
+        pos += 9
+    )
     {
-        return -1;
-    }
-
-    // "processorDDD"
-    // "processorsNNN"
-    // "processorsNNN_AA-BB"
-
-
-    if (pos > 0 && objectPath[pos-1] != '/')
-    {
-        // Directory not starting with "processor" e.g. "somenamewithprocessor"
-        return -1;
-    }
-
-    procDir = objectPath;
-
-    // Strip leading directory
-    if (pos > 0)
-    {
-        path = objectPath.substr(0, pos-1);
-        procDir = objectPath.substr(pos);
-    }
-
-    // Strip trailing local directory
-    pos = procDir.find('/');
-    if (pos != string::npos)
-    {
-        local = procDir.substr(pos+1);
-        procDir = procDir.substr(0, pos);
-    }
-
-    // Now procDir is e.g.
-    // - processor0
-    // - processors0
-    // - processorBananas
-
-    // Look for number after "processor"
-
-    fileName f(procDir.substr(9));
-
-    if (f.size() && f[0] == 's')
-    {
-        // "processsorsNNN"
-
-        f = f.substr(1);
-
-        // Detect "processorsNNN_AA-BB"
+        if (pos > 0 && objPath[pos-1] != '/')
         {
-            std::string::size_type fromStart = f.find("_");
-            std::string::size_type toStart = f.find("-");
-            if (fromStart != string::npos && toStart != string::npos)
-            {
-                string nProcsName(f.substr(0, fromStart));
-                string fromName(f.substr(fromStart+1, toStart-(fromStart+1)));
-                string toName(f.substr(toStart+1));
+            // Not start of string or after /processor
+            continue;
+        }
 
-                label groupEnd = -1;
-                if
+        // The parse point. One past 'processor'
+        size_t firstp = pos + 9;
+
+        // normal: 'processor(\d+)'
+        // plural: 'processors(\d+)'
+
+        const bool plural = (objPath[firstp] == 's');
+
+        if (plural)
+        {
+            ++firstp;  // Skip over the 's'
+        }
+        else if (!std::isdigit(objPath[firstp]))
+        {
+            // Non-plural version (uncollated) requires digits only
+            continue;
+        }
+
+        // The next slash indicates there is a local directory
+        slashLocal = objPath.find('/', firstp);
+
+        // The last parse point is the slash, or end of string
+        const size_t lastp =
+            (slashLocal == string::npos ? objPath.length() : slashLocal);
+
+        if (!std::isdigit(objPath[lastp-1]))
+        {
+            // Must end in a digit!
+            // This traps entries that are too short or look quite wrong
+            // and avoid a string to int conversion that will fail anyhow
+            continue;
+        }
+
+
+        // Match: '^processors(\d+)$'  -> nProcs
+
+        // Match: '^processors(\d+)_(\d+)-(\d+)$'
+        // \1 = nProcs
+        // \2 = beg processor group
+        // \3 = end processor group (inclusive)
+
+        if (plural)
+        {
+            int nProcsRead = 0;
+
+            if
+            (
+                parseProcsNumRange
                 (
-                    Foam::read(fromName.c_str(), groupStart)
-                 && Foam::read(toName.c_str(), groupEnd)
-                 && Foam::read(nProcsName.c_str(), nProcs)
+                    objPath.substr(firstp, lastp-firstp),
+                    nProcsRead,
+                    group
                 )
-                {
-                    groupSize = groupEnd-groupStart+1;
-                    return -1;
-                }
+            )
+            {
+                // Total number of processors
+                nProcs = nProcsRead;
+
+                // We are done!
+                break;
             }
         }
 
-        // Detect "processorsN"
-        label n;
-        if (Foam::read(f.c_str(), n))
+        // Single
+        // Match: '^processor(\d+)$'   -> proci
+
+        label proci = 0;
+        if
+        (
+            Foam::read(objPath.substr(firstp, lastp-firstp), proci)
+         && (proci >= 0)
+        )
         {
-            nProcs = n;
+            // Capture value of an individual processor
+            returnProci = proci;
+
+            // We are done!
+            break;
         }
-        return -1;
+
     }
-    else
+
+    if (pos != string::npos)
     {
-        // Detect "processorN"
-        label proci;
-        if (Foam::read(f.c_str(), proci))
+        // The split succeeded, extract the components.
+
+        // The leading directory
+        if (pos > 0)
         {
-            return proci;
+            path = objPath.substr(0, pos-1);
+        }
+
+        // The slash starting the trailing (local) directory
+        if (slashLocal != string::npos)
+        {
+            procDir = objPath.substr(pos, slashLocal-pos);
+            local = objPath.substr(slashLocal+1);
         }
         else
         {
-            return -1;
+            procDir = objPath.substr(pos);
         }
     }
+
+    return returnProci;
 }
 
 
 Foam::label Foam::fileOperation::detectProcessorPath(const fileName& fName)
 {
     fileName path, pDir, local;
-    label start, size, nProcs;
-    return splitProcessorPath(fName, path, pDir, local, start, size, nProcs);
+    procRangeType group;
+    label nProcs;
+    return splitProcessorPath(fName, path, pDir, local, group, nProcs);
 }
 
 
diff --git a/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.H b/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.H
index 2748bd580f..7fa6fb9348 100644
--- a/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.H
+++ b/src/OpenFOAM/global/fileOperations/fileOperation/fileOperation.H
@@ -99,26 +99,36 @@ public:
         };
         static const Enum<pathType> pathTypeNames_;
 
+
+        //- A dirIndex adds the path type and local offset to a fileName
         typedef Tuple2<fileName, Tuple2<pathType, label>> dirIndex;
         typedef List<dirIndex> dirIndexList;
 
+        //- For addressing a range of processors,
+        //- identical to UPstream::rangeType
+        typedef IntRange<int> procRangeType;
+
 
 protected:
 
-    // Protected data
+    // Protected Data
 
         //- Communicator to use
         const label comm_;
 
+        //- Distributed roots (parallel run)
+        bool distributed_;
+
         //- Detected processors directories
         mutable HashTable<dirIndexList> procsDirs_;
 
-        //- file-change monitor for all registered files
+        //- File-change monitor for all registered files
         mutable autoPtr<fileMonitor> monitorPtr_;
 
 
    // Protected Member Functions
 
+        //- Get or create fileMonitor singleton
         fileMonitor& monitor() const;
 
         //- Sort directory entries according to time value
@@ -148,7 +158,7 @@ protected:
         //  \return empty fileName if not found.
         virtual refPtr<dirIndexList> lookupProcessorsPath
         (
-            const fileName&
+            const fileName& objectPath
         ) const;
 
         //- Does ioobject exist. Is either a directory (empty name()) or
@@ -158,16 +168,16 @@ protected:
 
 public:
 
-    // Static data
+    // Static Data
 
         //- Return the processors directory name (usually "processors")
         static word processorsBaseDir;
 
-        //- Default fileHandler
+        //- Name of the default fileHandler
         static word defaultFileHandler;
 
 
-    // Public data types
+    // Public Data Types
 
         //- Runtime type information
         TypeName("fileOperation");
@@ -179,8 +189,12 @@ public:
 
     // Constructors
 
-        //- Construct from communicator
-        explicit fileOperation(const label comm);
+        //- Construct from communicator, optionally with distributed roots
+        explicit fileOperation
+        (
+            const label comm,
+            const bool distributedRoots = false
+        );
 
 
     // Declare run-time constructor selection table
@@ -213,6 +227,21 @@ public:
 
     // Member Functions
 
+        //- Distributed roots (parallel run)
+        bool distributed() const noexcept
+        {
+            return distributed_;
+        }
+
+        //- Set distributed roots on/off, return old value
+        bool distributed(bool on) noexcept
+        {
+            bool old(distributed_);
+            distributed_ = on;
+            return old;
+        }
+
+
         // OSSpecific equivalents
 
             //- Make directory
@@ -538,7 +567,8 @@ public:
             //- Operating on fileName: replace processorXXX with procDir
             fileName processorsPath(const fileName&, const word& procDir) const;
 
-            //- Split fileName into part before 'processor' and part after.
+            //- Split objectPath into part before 'processor' and part after.
+            //
             //  Returns -1 or processor number and optionally number
             //  of processors. Use with care.
             //  - path/"processor"+Foam::name(proci)/local reconstructs input
@@ -546,17 +576,16 @@ public:
             //    collated processors equivalence
             static label splitProcessorPath
             (
-                const fileName&,
+                const fileName& objectPath,
                 fileName& path,
                 fileName& procDir,
                 fileName& local,
-                label& groupStart,
-                label& groupSize,
+                procRangeType& group,
                 label& nProcs
             );
 
             //- Detect processor number from '/aa/bb/processorDDD/cc'
-            static label detectProcessorPath(const fileName&);
+            static label detectProcessorPath(const fileName& objPath);
 };
 
 
diff --git a/src/OpenFOAM/global/fileOperations/masterUncollatedFileOperation/masterUncollatedFileOperation.C b/src/OpenFOAM/global/fileOperations/masterUncollatedFileOperation/masterUncollatedFileOperation.C
index 64d99bee45..2dc64cb039 100644
--- a/src/OpenFOAM/global/fileOperations/masterUncollatedFileOperation/masterUncollatedFileOperation.C
+++ b/src/OpenFOAM/global/fileOperations/masterUncollatedFileOperation/masterUncollatedFileOperation.C
@@ -200,15 +200,16 @@ Foam::fileOperations::masterUncollatedFileOperation::filePathInfo
         if (io.time().processorCase())
         {
             refPtr<dirIndexList> pDirs(lookupProcessorsPath(io.objectPath()));
-            forAll(pDirs(), i)
+
+            for (const dirIndex& dirIdx : pDirs())
             {
-                const fileName& pDir = pDirs()[i].first();
+                const fileName& pDir = dirIdx.first();
                 fileName objPath =
                     processorsPath(io, io.instance(), pDir)
                    /io.name();
                 if (objPath != writePath && isFileOrDir(isFile, objPath))
                 {
-                    searchType = pDirs()[i].second().first();
+                    searchType = dirIdx.second().first();
                     procsDir = pDir;
                     return objPath;
                 }
@@ -272,9 +273,10 @@ Foam::fileOperations::masterUncollatedFileOperation::filePathInfo
                 (
                     lookupProcessorsPath(io.objectPath())
                 );
-                forAll(pDirs(), i)
+
+                for (const dirIndex& dirIdx : pDirs())
                 {
-                    const fileName& pDir = pDirs()[i].first();
+                    const fileName& pDir = dirIdx.first();
 
                     fileName fName
                     (
@@ -283,7 +285,7 @@ Foam::fileOperations::masterUncollatedFileOperation::filePathInfo
                     );
                     if (isFileOrDir(isFile, fName))
                     {
-                        switch (pDirs()[i].second().first())
+                        switch (dirIdx.second().first())
                         {
                             case fileOperation::PROCUNCOLLATED:
                             {
@@ -1435,9 +1437,9 @@ bool Foam::fileOperations::masterUncollatedFileOperation::exists
     // 2. Check processors/
     if (io.time().processorCase())
     {
-        forAll(pDirs, i)
+        for (const dirIndex& dirIdx : pDirs)
         {
-            const fileName& pDir = pDirs[i].first();
+            const fileName& pDir = dirIdx.first();
             fileName procPath =
                 processorsPath(io, io.instance(), pDir)
                /io.name();
@@ -1953,18 +1955,13 @@ Foam::fileOperations::masterUncollatedFileOperation::readStream
 
 
         // Analyse the file path (on (co)master) to see the processors type
+        // Note: this should really be part of filePath() which should return
+        // both file and index in file.
+
         fileName path, procDir, local;
-        label groupStart, groupSize, nProcs;
-        splitProcessorPath
-        (
-            fName,
-            path,
-            procDir,
-            local,
-            groupStart,
-            groupSize,
-            nProcs
-        );
+        procRangeType group;
+        label nProcs;
+        splitProcessorPath(fName, path, procDir, local, group, nProcs);
 
 
         if (!Pstream::parRun())
@@ -1981,12 +1978,10 @@ Foam::fileOperations::masterUncollatedFileOperation::readStream
                     << exit(FatalIOError);
             }
 
-            // Analyse the fileName for any processor subset. Note: this
-            // should really be part of filePath() which should return
-            // both file and index in file.
-            if (groupStart != -1 && groupSize > 0)
+            // The local rank (offset)
+            if (!group.empty())
             {
-                proci = proci-groupStart;
+                proci = proci - group.start();
             }
 
             if (debug)
@@ -2009,7 +2004,7 @@ Foam::fileOperations::masterUncollatedFileOperation::readStream
             // Are we reading from single-master file ('processors256') or
             // from multi-master files ('processors256_0-9')
             label readComm = -1;
-            if (groupStart != -1 && groupSize > 0)
+            if (!group.empty())
             {
                 readComm = comm_;
                 if (UPstream::master(comm_) && !isPtr && !fName.empty())
diff --git a/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.C b/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.C
index 0a33c54642..b930c685fe 100644
--- a/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.C
+++ b/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.C
@@ -119,12 +119,13 @@ Foam::fileName Foam::fileOperations::uncollatedFileOperation::filePathInfo
                     fileOperation::lookupAndCacheProcessorsPath
                     (
                         io.objectPath(),
-                        false
+                        false // No additional parallel synchronisation
                     )
                 );
-                forAll(pDirs(), i)
+
+                for (const dirIndex& dirIdx : pDirs())
                 {
-                    const fileName& pDir = pDirs()[i].first();
+                    const fileName& pDir = dirIdx.first();
                     fileName objPath =
                         processorsPath(io, io.instance(), pDir)
                        /io.name();
@@ -172,8 +173,8 @@ Foam::fileOperations::uncollatedFileOperation::lookupProcessorsPath
     const fileName& fName
 ) const
 {
-    // Do not use parallel synchronisation
-    return lookupAndCacheProcessorsPath(fName, false);
+    // No additional parallel synchronisation
+    return fileOperation::lookupAndCacheProcessorsPath(fName, false);
 }
 
 
@@ -592,20 +593,14 @@ Foam::fileOperations::uncollatedFileOperation::readStream
         // should really be part of filePath() which should return
         // both file and index in file.
         fileName path, procDir, local;
-        label groupStart, groupSize, nProcs;
-        splitProcessorPath
-        (
-            fName,
-            path,
-            procDir,
-            local,
-            groupStart,
-            groupSize,
-            nProcs
-        );
-        if (groupStart != -1 && groupSize > 0)
+        procRangeType group;
+        label nProcs;
+        splitProcessorPath(fName, path, procDir, local, group, nProcs);
+
+        // The local rank (offset)
+        if (!group.empty())
         {
-            proci = proci-groupStart;
+            proci = proci - group.start();
         }
 
         // Read data and return as stream
diff --git a/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.H b/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.H
index 496f64014e..3e2ec313e4 100644
--- a/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.H
+++ b/src/OpenFOAM/global/fileOperations/uncollatedFileOperation/uncollatedFileOperation.H
@@ -69,7 +69,7 @@ protected:
         ) const;
 
         //- Lookup name of processorsDDD using cache.
-        //  \not Do not use any parallel synchronisation
+        //  \note Do not use any parallel synchronisation
         //  \return empty fileName if not found.
         virtual refPtr<dirIndexList> lookupProcessorsPath
         (
@@ -85,7 +85,7 @@ public:
 
     // Constructors
 
-        //- Construct null
+        //- Default construct
         uncollatedFileOperation(bool verbose);