diff --git a/src/USER-MISC/fix_ave_correlate_long.cpp b/src/USER-MISC/fix_ave_correlate_long.cpp
index cf9e6c0dca..2aafe9bac0 100644
--- a/src/USER-MISC/fix_ave_correlate_long.cpp
+++ b/src/USER-MISC/fix_ave_correlate_long.cpp
@@ -23,6 +23,7 @@
 
 #include "fix_ave_correlate_long.h"
 
+#include "arg_info.h"
 #include "citeme.h"
 #include "compute.h"
 #include "error.h"
@@ -39,10 +40,8 @@
 using namespace LAMMPS_NS;
 using namespace FixConst;
 
-enum{COMPUTE,FIX,VARIABLE};
 enum{AUTO,UPPER,LOWER,AUTOUPPER,AUTOLOWER,FULL};
 
-
 static const char cite_fix_ave_correlate_long[] =
 "fix ave/correlate/long command:\n\n"
 "@Article{Ramirez10,\n"
@@ -82,33 +81,18 @@ FixAveCorrelateLong::FixAveCorrelateLong(LAMMPS * lmp, int narg, char **arg):
 
   int iarg = 5;
   while (iarg < narg) {
-    if (strncmp(arg[iarg],"c_",2) == 0 ||
-        strncmp(arg[iarg],"f_",2) == 0 ||
-        strncmp(arg[iarg],"v_",2) == 0) {
-      if (arg[iarg][0] == 'c') which[nvalues] = COMPUTE;
-      else if (arg[iarg][0] == 'f') which[nvalues] = FIX;
-      else if (arg[iarg][0] == 'v') which[nvalues] = VARIABLE;
 
-      int n = strlen(arg[iarg]);
-      char *suffix = new char[n];
-      strcpy(suffix,&arg[iarg][2]);
+    ArgInfo argi(arg[iarg]);
+    if (argi.get_type() == ArgInfo::NONE) break;
+    if ((argi.get_type() == ArgInfo::UNKNOWN) || (argi.get_dim() > 1))
+      error->all(FLERR,"Illegal fix ave/correlate/long command");
 
-      char *ptr = strchr(suffix,'[');
-      if (ptr) {
-        if (suffix[strlen(suffix)-1] != ']')
-          error->all(FLERR,"Illegal fix ave/correlate/long command");
-        argindex[nvalues] = atoi(ptr+1);
-        *ptr = '\0';
-      } else argindex[nvalues] = 0;
+    which[nvalues] = argi.get_type();
+    argindex[nvalues] = argi.get_index1();
+    ids[nvalues] = argi.copy_name();
 
-      n = strlen(suffix) + 1;
-      ids[nvalues] = new char[n];
-      strcpy(ids[nvalues],suffix);
-      delete [] suffix;
-
-      nvalues++;
-      iarg++;
-    } else break;
+    nvalues++;
+    iarg++;
   }
 
   // optional args
@@ -204,7 +188,7 @@ FixAveCorrelateLong::FixAveCorrelateLong(LAMMPS * lmp, int narg, char **arg):
     error->all(FLERR,"Illegal fix ave/correlate/long command");
 
   for (int i = 0; i < nvalues; i++) {
-    if (which[i] == COMPUTE) {
+    if (which[i] == ArgInfo::COMPUTE) {
       int icompute = modify->find_compute(ids[i]);
       if (icompute < 0)
         error->all(FLERR,"Compute ID for fix ave/correlate/long does not exist");
@@ -218,7 +202,7 @@ FixAveCorrelateLong::FixAveCorrelateLong(LAMMPS * lmp, int narg, char **arg):
         error->all(FLERR,"Fix ave/correlate/long compute vector "
                    "is accessed out-of-range");
 
-    } else if (which[i] == FIX) {
+    } else if (which[i] == ArgInfo::FIX) {
       int ifix = modify->find_fix(ids[i]);
       if (ifix < 0)
         error->all(FLERR,"Fix ID for fix ave/correlate/long does not exist");
@@ -233,7 +217,7 @@ FixAveCorrelateLong::FixAveCorrelateLong(LAMMPS * lmp, int narg, char **arg):
         error->all(FLERR,"Fix for fix ave/correlate/long "
                    "not computed at compatible time");
 
-    } else if (which[i] == VARIABLE) {
+    } else if (which[i] == ArgInfo::VARIABLE) {
       int ivariable = input->variable->find(ids[i]);
       if (ivariable < 0)
         error->all(FLERR,"Variable name for fix ave/correlate/long does not exist");
@@ -377,19 +361,19 @@ void FixAveCorrelateLong::init()
   // set current indices for all computes,fixes,variables
 
   for (int i = 0; i < nvalues; i++) {
-    if (which[i] == COMPUTE) {
+    if (which[i] == ArgInfo::COMPUTE) {
       int icompute = modify->find_compute(ids[i]);
       if (icompute < 0)
         error->all(FLERR,"Compute ID for fix ave/correlate/long does not exist");
       value2index[i] = icompute;
 
-    } else if (which[i] == FIX) {
+    } else if (which[i] == ArgInfo::FIX) {
       int ifix = modify->find_fix(ids[i]);
       if (ifix < 0)
         error->all(FLERR,"Fix ID for fix ave/correlate/long does not exist");
       value2index[i] = ifix;
 
-    } else if (which[i] == VARIABLE) {
+    } else if (which[i] == ArgInfo::VARIABLE) {
       int ivariable = input->variable->find(ids[i]);
       if (ivariable < 0)
         error->all(FLERR,"Variable name for fix ave/correlate/long does not exist");
@@ -441,7 +425,7 @@ void FixAveCorrelateLong::end_of_step()
 
     // invoke compute if not previously invoked
 
-    if (which[i] == COMPUTE) {
+    if (which[i] == ArgInfo::COMPUTE) {
       Compute *compute = modify->compute[m];
 
       if (argindex[i] == 0) {
@@ -460,7 +444,7 @@ void FixAveCorrelateLong::end_of_step()
 
     // access fix fields, guaranteed to be ready
 
-    } else if (which[i] == FIX) {
+    } else if (which[i] == ArgInfo::FIX) {
       if (argindex[i] == 0)
         scalar = modify->fix[m]->compute_scalar();
       else
@@ -468,7 +452,7 @@ void FixAveCorrelateLong::end_of_step()
 
     // evaluate equal-style variable
 
-    } else if (which[i] == VARIABLE)
+    } else if (which[i] == ArgInfo::VARIABLE)
       scalar = input->variable->compute_equal(m);
 
     values[i] = scalar;
diff --git a/src/thermo.cpp b/src/thermo.cpp
index 6d806b65e3..649851c9e4 100644
--- a/src/thermo.cpp
+++ b/src/thermo.cpp
@@ -18,6 +18,7 @@
 #include "thermo.h"
 
 #include "angle.h"
+#include "arg_info.h"
 #include "atom.h"
 #include "bond.h"
 #include "comm.h"
@@ -877,35 +878,25 @@ void Thermo::parse_fields(char *str)
     } else if (word == "cellgamma") {
       addfield("CellGamma",&Thermo::compute_cellgamma,FLOAT);
 
-    // compute value = c_ID, fix value = f_ID, variable value = v_ID
-    // count trailing [] and store int arguments
+      // compute value = c_ID, fix value = f_ID, variable value = v_ID
+      // count trailing [] and store int arguments
 
-    } else if ((word.substr(0, 2) == "c_") || (word.substr(0, 2) == "f_") ||
-               (word.substr(0, 2) == "v_")) {
+    } else {
+      ArgInfo argi(word);
 
-      int n =  word.length() - 1;
-      char *id = new char[n];
-      strcpy(id, &word.c_str()[2]);
+      if ((argi.get_type() == ArgInfo::UNKNOWN)
+          || (argi.get_type() == ArgInfo::NONE)
+          || (argi.get_dim() > 2))
+        error->all(FLERR,"Unknown keyword in thermo_style custom command");
 
-      // parse zero or one or two trailing brackets from ID
+      // process zero or one or two trailing brackets
       // argindex1,argindex2 = int inside each bracket pair, 0 if no bracket
 
-      char *ptr = strchr(id,'[');
-      if (ptr == nullptr) argindex1[nfield] = argindex2[nfield] = 0;
-      else {
-        *ptr = '\0';
-        argindex1[nfield] =
-          (int) input->variable->int_between_brackets(ptr,0);
-        ptr++;
-        if (*ptr == '[') {
-          argindex2[nfield] =
-            (int) input->variable->int_between_brackets(ptr,0);
-          ptr++;
-        } else argindex2[nfield] = 0;
-      }
+      argindex1[nfield] = argi.get_index1();
+      argindex2[nfield] = (argi.get_dim() > 1) ? argi.get_index2() : 0;
 
-      if (word[0] == 'c') {
-        n = modify->find_compute(id);
+      if (argi.get_type() == ArgInfo::COMPUTE) {
+        int n = modify->find_compute(argi.get_name());
         if (n < 0) error->all(FLERR,"Could not find thermo custom compute ID");
         if (argindex1[nfield] == 0 && modify->compute[n]->scalar_flag == 0)
           error->all(FLERR,"Thermo compute does not compute scalar");
@@ -927,15 +918,15 @@ void Thermo::parse_fields(char *str)
         }
 
         if (argindex1[nfield] == 0)
-          field2index[nfield] = add_compute(id, SCALAR);
+          field2index[nfield] = add_compute(argi.get_name(), SCALAR);
         else if (argindex2[nfield] == 0)
-          field2index[nfield] = add_compute(id, VECTOR);
+          field2index[nfield] = add_compute(argi.get_name(), VECTOR);
         else
-          field2index[nfield] = add_compute(id, ARRAY);
+          field2index[nfield] = add_compute(argi.get_name(), ARRAY);
         addfield(word.c_str(), &Thermo::compute_compute, FLOAT);
 
-      } else if (word[0] == 'f') {
-        n = modify->find_fix(id);
+      } else if (argi.get_type() == ArgInfo::FIX) {
+        int n = modify->find_fix(argi.get_name());
         if (n < 0) error->all(FLERR,"Could not find thermo custom fix ID");
         if (argindex1[nfield] == 0 && modify->fix[n]->scalar_flag == 0)
           error->all(FLERR,"Thermo fix does not compute scalar");
@@ -956,11 +947,11 @@ void Thermo::parse_fields(char *str)
             error->all(FLERR,"Thermo fix array is accessed out-of-range");
         }
 
-        field2index[nfield] = add_fix(id);
+        field2index[nfield] = add_fix(argi.get_name());
         addfield(word.c_str(), &Thermo::compute_fix, FLOAT);
 
-      } else if (word[0] == 'v') {
-        n = input->variable->find(id);
+      } else if (argi.get_type() == ArgInfo::VARIABLE) {
+        int n = input->variable->find(argi.get_name());
         if (n < 0)
           error->all(FLERR,"Could not find thermo custom variable name");
         if (argindex1[nfield] == 0 && input->variable->equalstyle(n) == 0)
@@ -972,14 +963,10 @@ void Thermo::parse_fields(char *str)
         if (argindex2[nfield])
           error->all(FLERR,"Thermo custom variable cannot have two indices");
 
-        field2index[nfield] = add_variable(id);
+        field2index[nfield] = add_variable(argi.get_name());
         addfield(word.c_str(), &Thermo::compute_variable, FLOAT);
       }
-
-      delete [] id;
-
-    } else error->all(FLERR,"Unknown keyword in thermo_style custom command");
-
+    }
   }
 }