profile: add --regex option to match packages

src/nix/profile-remove.md

@@ -11,7 +11,7 @@ R""(
 * Remove all packages:
 
   ```console
-  # nix profile remove '.*'
+  # nix profile remove --regex '.*'
   ```
 
 * Remove a package by store path:

src/nix/profile-upgrade.md

@@ -6,7 +6,7 @@ R""(
   reference:
 
   ```console
-  # nix profile upgrade '.*'
+  # nix profile upgrade --regex '.*'
   ```
 
 * Upgrade a specific package by name:

src/nix/profile.cc

@@ -508,7 +508,15 @@ public:
 
     MixProfileElementMatchers()
     {
-        expectArgs(ExpectedArg {
+        addFlag({
+            .longName = "regex",
+            .description = "A regular expression to match one or more packages in the profile.",
+            .labels = {"pattern"},
+            .handler = {[this](std::string arg) {
+                _matchers.push_back(createRegexMatcher(arg));
+            }},
+        });
+        expectArgs({
             .label = "elements",
             .optional = true,
             .handler = {[this](std::vector<std::string> args) {