diff --git a/include/ruby/internal/core/rregexp.h b/include/ruby/internal/core/rregexp.h
index 3c33f12860..cf54a399f1 100644
--- a/include/ruby/internal/core/rregexp.h
+++ b/include/ruby/internal/core/rregexp.h
@@ -28,7 +28,20 @@
 #include "ruby/internal/value.h"
 #include "ruby/internal/value_type.h"
 
+/**
+ * Convenient casting macro.
+ *
+ * @param   obj  An object, which is in fact an ::RRegexp.
+ * @return  The passed object casted to ::RRegexp.
+ */
 #define RREGEXP(obj)     RBIMPL_CAST((struct RRegexp *)(obj))
+
+/**
+ * Convenient accessor macro.
+ *
+ * @param   obj  An object, which is in fact an ::RRegexp.
+ * @return  The passed object's pattern buffer.
+ */
 #define RREGEXP_PTR(obj) (RREGEXP(obj)->ptr)
 /** @cond INTERNAL_MACRO */
 #define RREGEXP_SRC      RREGEXP_SRC
@@ -37,17 +50,55 @@
 #define RREGEXP_SRC_END  RREGEXP_SRC_END
 /** @endcond */
 
-struct re_patter_buffer; /* a.k.a. OnigRegexType, defined in onigmo.h */
+struct re_patter_buffer;  /* a.k.a. OnigRegexType, defined in onigmo.h */
 
+/**
+ * Ruby's regular expression.   A regexp is compiled into  its own intermediate
+ * representation.  This  one holds that  info.  Regexp "match"  operation then
+ * executes that IR.
+ */
 struct RRegexp {
+
+    /** Basic part, including flags and class. */
     struct RBasic basic;
+
+    /**
+     * The pattern buffer.   This is a quasi-opaque struct  that holds compiled
+     * intermediate representation of the regular expression.
+     *
+     * @note  Compilation of a regexp could be delayed until actual match.
+     */
     struct re_pattern_buffer *ptr;
+
+    /** Source code of this expression. */
     const VALUE src;
+
+    /**
+     * Reference count.  A  regexp match can take extraordinarily  long time to
+     * run.  Ruby's  regular expression is  heavily extended and not  a regular
+     * language any  longer; runs in NP-time  in practice.  Now, Ruby  also has
+     * threads and GVL.  In order to prevent long GVL lockup, our regexp engine
+     * can release it on occasions.  This means that multiple threads can touch
+     * a regular expressions at once.  That  itself is okay.  But their cleanup
+     * phase shall wait for all  the concurrent runs, to prevent use-after-free
+     * situation.  This field is used to  count such threads that are executing
+     * this particular pattern buffer.
+     *
+     * @warning  Of course, touching this field from extension libraries causes
+     *           catastrophic effects.  Just leave it.
+     */
     unsigned long usecnt;
 };
 
 RBIMPL_ATTR_PURE_UNLESS_DEBUG()
 RBIMPL_ATTR_ARTIFICIAL()
+/**
+ * Convenient getter function.
+ *
+ * @param[in]  rexp  The regular expression in question.
+ * @return     The source code of the regular expression.
+ * @pre        `rexp` must be of ::RRegexp.
+ */
 static inline VALUE
 RREGEXP_SRC(VALUE rexp)
 {
@@ -59,6 +110,17 @@ RREGEXP_SRC(VALUE rexp)
 
 RBIMPL_ATTR_PURE_UNLESS_DEBUG()
 RBIMPL_ATTR_ARTIFICIAL()
+/**
+ * Convenient getter function.
+ *
+ * @param[in]  rexp  The regular expression in question.
+ * @return     The source code of the regular expression, in C's string.
+ * @pre        `rexp` must be of ::RRegexp.
+ *
+ * @internal
+ *
+ * It seems nobody uses this function in the wild.  Subject to hide?
+ */
 static inline char *
 RREGEXP_SRC_PTR(VALUE rexp)
 {
@@ -67,6 +129,17 @@ RREGEXP_SRC_PTR(VALUE rexp)
 
 RBIMPL_ATTR_PURE_UNLESS_DEBUG()
 RBIMPL_ATTR_ARTIFICIAL()
+/**
+ * Convenient getter function.
+ *
+ * @param[in]  rexp  The regular expression in question.
+ * @return     The length of the source code of the regular expression.
+ * @pre        `rexp` must be of ::RRegexp.
+ *
+ * @internal
+ *
+ * It seems nobody uses this function in the wild.  Subject to hide?
+ */
 static inline long
 RREGEXP_SRC_LEN(VALUE rexp)
 {
@@ -75,6 +148,17 @@ RREGEXP_SRC_LEN(VALUE rexp)
 
 RBIMPL_ATTR_PURE_UNLESS_DEBUG()
 RBIMPL_ATTR_ARTIFICIAL()
+/**
+ * Convenient getter function.
+ *
+ * @param[in]  rexp  The regular expression in question.
+ * @return     The end of the source code of the regular expression.
+ * @pre        `rexp` must be of ::RRegexp.
+ *
+ * @internal
+ *
+ * It seems nobody uses this function in the wild.  Subject to hide?
+ */
 static inline char *
 RREGEXP_SRC_END(VALUE rexp)
 {