Do not compile regexes on each run (#3949)

* Do not compile regexes on each run

- Keep a cache of already compiled regexes (they will always be the same)
- Alphanumeric strings an be simply matched for `==` instead of using RE.

* Deduplicate loop as per CR

go_expvar/datadog_checks/go_expvar/go_expvar.py

@@ -63,6 +63,7 @@
 class GoExpvar(AgentCheck):
     def __init__(self, name, init_config, agentConfig, instances=None):
         AgentCheck.__init__(self, name, init_config, agentConfig, instances)
+        self._regexes = {}
         self._last_gc_count = defaultdict(int)
 
     def _get_data(self, url, instance):
@@ -227,16 +228,26 @@ def deep_get(self, content, keys, traversed_path=None):
             return [(traversed_path, content)]
 
         key = keys[0]
-        regex = "".join(["^", key, "$"])
-        try:
-            key_rex = re.compile(regex)
-        except Exception:
-            self.warning("Cannot compile regex: %s" % regex)
-            return []
+        if key.isalnum():
+            # key is not a regex, simply match for equality
+            matcher = key.__eq__
+        else:
+            # key might be a regex
+            key_regex = self._regexes.get(key)
+            if key_regex is None:
+                # we don't have it cached, compile it
+                regex = "^{}$".format(key)
+                try:
+                    key_regex = re.compile(regex)
+                except Exception:
+                    self.warning("Cannot compile regex: %s" % regex)
+                    return []
+                self._regexes[key] = key_regex
+            matcher = key_regex.match
 
         results = []
         for new_key, new_content in self.items(content):
-            if key_rex.match(new_key):
+            if matcher(new_key):
                 results.extend(self.deep_get(new_content, keys[1:], traversed_path + [str(new_key)]))
         return results