Add more detailed explanations to control-flow RegexOpcode values

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexOpcode.cs

@@ -117,54 +117,169 @@ internal enum RegexOpcode
         UpdateBumpalong = 46,
 
         // Primitive control structures
-        // TODO: Figure out what these comments mean / what these control structures actually do :)
 
-        /// <summary>back     jump            straight first.</summary>
+        /// <summary>Lazy branch in an alternation or conditional construct.</summary>
+        /// <remarks>
+        /// On first execution, the opcode records the current input position (via the tracking stack) and continues straight
+        /// without taking the jump. When the matching that follows fails, backtracking will occur and the saved position is restored,
+        /// at which point the interpreter will jump to the alternative branch (using the patched jump offset in operand 0).
+        /// This opcode is used to implement alternation in a non-greedy (lazy) manner.
+        /// </remarks>
         Lazybranch = 23,
-        /// <summary>back     jump            branch first for loop.</summary>
+
+        /// <summary>Branch in a quantified loop that uses a saved mark to decide whether to repeat or exit.</summary>
+        /// <remarks>
+        /// When executed, this opcode pops a previously saved input mark (from a <see cref="Setmark"/> or <see cref="Nullmark"/>)
+        /// and compares it to the current input position. If the loop's inner expression has consumed input (non-empty match), it
+        /// pushes updated state (saving the old mark and the current position) and jumps back (via the jump offset in operand 0)
+        /// to repeat the loop. If no progress has been made (empty match), it records state for backtracking and proceeds.
+        /// This opcode is used for greedy (non-lazy) quantified loops when no explicit counter is needed.
+        /// </remarks>
         Branchmark = 24,
-        /// <summary>back     jump            straight first for loop.</summary>
+
+        /// <summary>Lazy branch in a quantified loop that uses a saved mark.</summary>
+        /// <remarks>
+        /// Similar in spirit to <see cref="Branchmark"/>, this opcode is used for lazy loops.
+        /// It initially does not jump back to repeat the loop, preferring to let the overall match continue.
+        /// However, it saves the loop state so that if subsequent matching fails, backtracking will re-enter the loop body.
+        /// Special care is taken to handle empty matches so as to avoid infinite loops.
+        /// </remarks>
         Lazybranchmark = 25,
-        /// <summary>back     val             set counter, null mark.</summary>
+
+        /// <summary>Initialize the loop counter for a quantifier when the minimum repetition is zero.</summary>
+        /// <remarks>
+        /// For quantified constructs with a minimum of zero (<see cref="RegexNode.M"/> == 0), this opcode pushes a counter
+        /// value (-1) along with a marker (implicitly indicating no match so far) onto the grouping stack. The operand (always 0
+        /// in this case) is used in later comparisons within a <see cref="Branchcount"/> or <see cref="Lazybranchcount"/> opcode.
+        /// </remarks>
         Nullcount = 26,
-        /// <summary>back     val             set counter, make mark</summary>
+
+        /// <summary>Initialize the loop counter for a quantifier with a positive minimum.</summary>
+        /// <remarks>
+        /// When the quantifier requires at least one match (M > 0), this opcode pushes the current input position as a marker and a
+        /// counter value computed as (1 - M) onto the grouping stack. This counter will be adjusted in subsequent loop iterations
+        /// (via <see cref="Branchcount"/> or <see cref="Lazybranchcount"/>) to decide whether the loop should continue.
+        /// </remarks>
         Setcount = 27,
-        /// <summary>back     jump,limit      branch++ if zero&lt;=c&lt;limit.</summary>
+
+        /// <summary>Greedy counted branch for quantified loops.</summary>
+        /// <remarks>
+        /// This opcode is used for quantified loops that require a counter. When executed, it pops the previously stored marker and counter
+        /// from the grouping stack, computes the difference between the current input position and the marker, and compares the counter
+        /// against a limit (given in operand 1). If the counter indicates that more iterations are allowed (and the inner expression consumed
+        /// input), it increments the counter, updates the marker with the new position, and jumps (via the jump offset in operand 0) to
+        /// repeat the loop. Otherwise, the interpreter continues straight. On backtracking, the previous state is restored so that a decreased
+        /// count may be tried.
+        /// </remarks>
         Branchcount = 28,
-        /// <summary>back     jump,limit      same, but straight first.</summary>
+
+        /// <summary>Lazy counted branch for quantified loops.</summary>
+        /// <remarks>
+        /// This opcode is the lazy counterpart to <see cref="Branchcount"/>. It is used in quantified loops that use a counter and prefer
+        /// to exit the loop as early as possible. On initial execution it will choose the straight path (i.e. not repeating the loop) if
+        /// the counter is nonnegative, but if the inner expression consumed input and the counter is below the maximum (given in operand 1),
+        /// it will re-enter the loop on backtracking.
+        /// </remarks>
         Lazybranchcount = 29,
-        /// <summary>back                     save position.</summary>
+
+        /// <summary>Push a null marker into the grouping stack for quantifiers with a minimum of zero when no explicit counter is needed.</summary>
+        /// <remarks>
+        /// This opcode is similar to <see cref="Nullcount"/> but is used in cases where the quantified construct does not require counting;
+        /// it pushes a marker value (-1) onto the grouping stack to record the starting position. On backtracking, the marker is simply removed.
+        /// </remarks>
         Nullmark = 30,
-        /// <summary>back                     save position.</summary>
+
+        /// <summary>Push the current input position onto the grouping stack.</summary>
+        /// <remarks>
+        /// Used by grouping constructs (for capturing or to detect empty matches in loops), this opcode saves the current input position
+        /// so that later the interpreter can compare it to the current position to decide whether progress was made. It is the non-counting
+        /// counterpart to <see cref="Setcount"/>.
+        /// </remarks>
         Setmark = 31,
-        /// <summary>back     group           define group.</summary>
+
+        /// <summary>Completes a capturing group.</summary>
+        /// <remarks>
+        /// When executed, this opcode pops a previously saved marker (the start position of the group) from the grouping stack and uses the
+        /// current input position as the end position. Operand 0 specifies the capture slot number. If operand 1 is not -1 then a prior capture
+        /// must have been made and a transfer of capture is performed. On backtracking, the capture is undone.
+        /// </remarks>
         Capturemark = 32,
-        /// <summary>back                     recall position.</summary>
+
+        /// <summary>Recall a previously saved marker.</summary>
+        /// <remarks>
+        /// This opcode restores the input position from a marker saved on the grouping stack (typically via a <see cref="Setmark"/> or
+        /// <see cref="Nullmark"/>). It is used in lookaround constructs to revert the input position to the point where the lookaround began.
+        /// On backtracking, the marker is re-pushed onto the grouping stack.
+        /// </remarks>
         Getmark = 33,
-        /// <summary>back                     save backtrack state.</summary>
+
+        /// <summary>Mark the beginning of a non-backtracking / atomic region.</summary>
+        /// <remarks>
+        /// This opcode is used at the start of constructs that must not be re-entered on backtracking (such as lookahead/lookbehind or atomic groups).
+        /// It saves the current backtracking state (including the current tracking and crawl positions) onto the grouping stack.
+        /// When the region is later exited (by <see cref="Forejump"/>) the saved state is used to prevent further backtracking into the region.
+        /// </remarks>
         Setjump = 34,
-        /// <summary>zap back to saved state.</summary>
+
+        /// <summary>Restore state for a non-backtracking / atomic region on backtracking.</summary>
+        /// <remarks>
+        /// Used in negative lookaround constructs, this opcode pops the saved backtracking and capture state (stored by a prior <see cref="Setjump"/>)
+        /// and erases any changes made within the non-backtracking region. It thereby restores the state to what it was before entering the region.
+        /// </remarks>
         Backjump = 35,
-        /// <summary>zap backtracking state.</summary>
+
+        /// <summary>Finalize a non-backtracking / atomic region.</summary>
+        /// <remarks>
+        /// This opcode is used at the end of lookaround or atomic group constructs to commit to the current matching path.
+        /// It pops the saved state from the grouping stack (stored by <see cref="Setjump"/>), updates the tracking pointer (thereby
+        /// discarding any backtracking state from within the region), and then continues execution. On backtracking from such a region,
+        /// a variant of this opcode will undo any captures made.
+        /// </remarks>
         Forejump = 36,
-        /// <summary>Backtrack if ref undefined.</summary>
+
+        /// <summary>Test whether a particular backreference has already matched.</summary>
+        /// <remarks>
+        /// Operand 0 is the capture group number to test. When executed, if the specified group has not captured any text,
+        /// the match fails and control transfers to backtracking. Otherwise, execution continues. This opcode is used in conditional
+        /// constructs where a branch is taken only if a given capture exists.
+        /// </remarks>
         TestBackreference = 37,
-        /// <summary>jump            just go.</summary>
+
+        /// <summary>Unconditional jump.</summary>
+        /// <remarks>
+        /// Operand 0 holds the target offset. When executed, the interpreter jumps unconditionally to that location.
+        /// This opcode is used to implement control flow for alternation and loop constructs.
+        /// </remarks>
         Goto = 38,
-        /// <summary>done!</summary>
+
+        /// <summary>Halt the interpreter.</summary>
+        /// <remarks>
+        /// This opcode marks the end of the opcode stream. When reached, the matching process terminates and the result
+        /// (whether a match was found) is returned.
+        /// </remarks>
         Stop = 40,
 
         // Modifiers for alternate modes
 
         /// <summary>Mask to get unmodified ordinary operator.</summary>
         OperatorMask = 63,
+
         /// <summary>Indicates that we're reverse scanning.</summary>
         RightToLeft = 64,
+
         /// <summary>Indicates that we're backtracking.</summary>
         Backtracking = 128,
+
         /// <summary>Indicates that we're backtracking on a second branch.</summary>
+        /// <remarks>
+        /// In patterns with alternations or complex quantifiers, multiple backtracking paths may be available.
+        /// This flag marks opcodes that are being processed on an alternate (or secondary) branch during backtracking,
+        /// as opposed to the primary branch. The interpreter uses this flag to apply specialized state restoration
+        /// or branch-selection logic when reverting from one branch to another.
+        /// </remarks>
         BacktrackingSecond = 256,
-        /// <summary>Indicates that we're case-insensitive</summary>
+
+        /// <summary>Indicates that we're case-insensitive.</summary>
         CaseInsensitive = 512,
     }
 }