Settings filtering via regular expressions (#717)

* Settings filtering via regular expressions

Implements only the most basic - and the most
useful - special characters - ^$.*

If the search string does not contain a special
character, it is interpreted as before.  Otherwise
the match is either more strict if anchored by
^ or $, or less strict if it contains a . wildcard
or a * repetition.

* Commentary

* Eliminated . metacharacter

Grbl_Esp32/src/ProcessSettings.cpp

@@ -1,5 +1,6 @@
 #include "Grbl.h"
 #include <map>
+#include "Regex.h"
 
 // WG Readable and writable as guest
 // WU Readable and writable as user and admin
@@ -565,19 +566,13 @@ Error do_command_or_setting(const char* key, char* value, WebUI::AuthenticationL
     Error retval = Error::InvalidStatement;
     if (!value) {
         auto lcKey = String(key);
-        // We allow the key string to begin with *, which we remove.
-        // This lets us look at X axis settings with $*x.
-        // $x by itself is the disable alarm lock command
-        if (lcKey.startsWith("*")) {
-            lcKey.remove(0, 1);
-        }
         lcKey.toLowerCase();
         bool found = false;
         for (Setting* s = Setting::List; s; s = s->next()) {
             auto lcTest = String(s->getName());
             lcTest.toLowerCase();
 
-            if (lcTest.indexOf(lcKey) >= 0) {
+            if (regexMatch(lcKey.c_str(), lcTest.c_str())) {
                 const char* displayValue = auth_failed(s, value, auth_level) ? "<Authentication required>" : s->getStringValue();
                 show_setting(s->getName(), displayValue, NULL, out);
                 found = true;

Grbl_Esp32/src/Regex.cpp

@@ -0,0 +1,60 @@
+// Simple regular expression matcher from Rob Pike per
+// https://www.cs.princeton.edu/courses/archive/spr09/cos333/beautiful.html
+
+//    c    matches any literal character c
+//    ^    matches the beginning of the input string
+//    $    matches the end of the input string
+//    *    matches zero or more occurrences of any character
+
+// The code therein has been reformatted into the style used in this
+// project while replacing ints used as flags by bools.  The regex
+// syntax was changed by omitting '.' and making '*' equivalent to
+// ".*".  This regular expression matcher is for matching setting
+// names, where arbitrary repetion of literal characters is
+// unlikely.  Literal character repetition is most useful for
+// skipping whitespace, which does not occur in setting names.  The
+// "bare * wildcard" is similar to filename wildcarding in many shells
+// and CLIs.
+
+static bool matchHere(const char* regexp, const char* text);
+
+// matchStar - search for *regexp at beginning of text
+static bool matchStar(const char* regexp, const char* text) {
+    do {
+        if (matchHere(regexp, text)) {
+            return true;
+        }
+    } while (*text++ != '\0');
+    return false;
+}
+
+// matchHere - search for regex at beginning of text
+static bool matchHere(const char* regexp, const char* text) {
+    if (regexp[0] == '\0') {
+        return true;
+    }
+    if (regexp[0] == '*') {
+        return matchStar(regexp + 1, text);
+    }
+    if (regexp[0] == '$' && regexp[1] == '\0') {
+        return *text == '\0';
+    }
+    if (*text != '\0' && (regexp[0] == *text)) {
+        return matchHere(++regexp, ++text);
+    }
+    return false;
+}
+
+// match - search for regular expression anywhere in text
+// Returns true if text contains the regular expression regexp
+bool regexMatch(const char* regexp, const char* text) {
+    if (regexp[0] == '^') {
+        return matchHere(++regexp, text);
+    }
+    do {
+        if (matchHere(regexp, text)) {
+            return true;
+        }
+    } while (*text++ != '\0');
+    return false;
+}

Grbl_Esp32/src/Regex.h

@@ -0,0 +1,5 @@
+// Simple regular expression matcher.
+// See Regex.cpp for attribution, description and discussion
+
+// Returns true if text contains the regular expression regexp
+bool regexMatch(const char* regexp, const char* text);