[user@localhost]$ perl -e "use re 'debug'; my $s = q[I(2,3).I.I]; my $call = qr[I (?: \(\d (?:, \d)*\) )?]x; my $name = qr[ (?:$call \.)* (I) ]x; print qq(Matched $1.\n) if $s =~ m/$name/x;"

Compiling REx "I (?: \(\d (?:, \d)*\) )?"
Final program:
   1: EXACT <I> (3)
   3: CURLYX[0]{0,1} (18)
   5:   EXACT <(> (7)
   7:   POSIXU[\d] (8)
   8:   CURLYM[0]{0,INFTY} (15)
  10:     EXACT <,> (12)
  12:     POSIXU[\d] (13)
  13:     SUCCEED (0)
  14:   NOTHING (15)
  15:   EXACT <)> (17)
  17: WHILEM (0)
  18: NOTHING (19)
  19: END (0)
anchored "I" at 0..0 (checking anchored) minlen 1
Compiling REx " (?:(?^x:I (?: \(\d (?:, \d)*\) )?) \.)* (I) "
synthetic stclass "ANYOF[I]".
Final program:
   1: CURLYX[0]{0,INFTY} (24)
   3:   EXACT <I> (5)
   5:   CURLYX[0]{0,1} (20)
   7:     EXACT <(> (9)
   9:     POSIXU[\d] (10)
  10:     CURLYM[0]{0,INFTY} (17)
  12:       EXACT <,> (14)
  14:       POSIXU[\d] (15)
  15:       SUCCEED (0)
  16:     NOTHING (17)
  17:     EXACT <)> (19)
  19:   WHILEM (0)
  20:   NOTHING (21)
  21:   EXACT <.> (23)
  23: WHILEM[1/3] (0)
  24: NOTHING (25)
  25: OPEN1 (27)
  27:   EXACT <I> (29)
  29: CLOSE1 (31)
  31: END (0)
floating "I" at 0..2147483647 (checking floating) stclass ANYOF[I] minlen 1
Matching REx " (?:(?^x:I (?: \(\d (?:, \d)*\) )?) \.)* (I) " against "I(2,3).I.I"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..10] gave 0
  Found floating substr "I" at offset 0 (rx_origin now 0)...
  (multiline anchor test skipped)
  looking for class: start_shift: 0 check_at: 0 rx_origin: 0 endpos: 1
  Does not contradict STCLASS...
Intuit: Successfully guessed: match at offset 0
Matching stclass ANYOF[I] against "I(2,3).I.I" (10 bytes)
   0 <> <I(2,3).I.I>         |   0| 1:CURLYX[0]{0,INFTY}(24)
   0 <> <I(2,3).I.I>         |   1|  23:WHILEM[1/3](0)
                             |   1|  WHILEM: matched 0 out of 0..32767
   0 <> <I(2,3).I.I>         |   2|   3:EXACT <I>(5)
   1 <I> <(2,3).I.I>         |   2|   5:CURLYX[0]{0,1}(20)
   1 <I> <(2,3).I.I>         |   3|    19:WHILEM(0)
                             |   3|    WHILEM: matched 0 out of 0..1
   1 <I> <(2,3).I.I>         |   4|     7:EXACT <(>(9)
   2 <I(> <2,3).I.I>         |   4|     9:POSIXU[\d](10)
   3 <I(2> <,3).I.I>         |   4|     10:CURLYM[0]{0,INFTY}(17)
   3 <I(2> <,3).I.I>         |   5|      12:EXACT <,>(14)
   4 <I(2,> <3).I.I>         |   5|      14:POSIXU[\d](15)
   5 <I(2,3> <).I.I>         |   5|      15:SUCCEED(0)
                             |   5|      SUCCEED: subpattern success...
                             |   4|     CURLYM now matched 1 times, len=2...
   5 <I(2,3> <).I.I>         |   5|      12:EXACT <,>(14)
                             |   5|      failed...
                             |   4|     CURLYM trying tail with matches=1...
   5 <I(2,3> <).I.I>         |   5|      17:EXACT <)>(19)
   6 <I(2,3)> <.I.I>         |   5|      19:WHILEM(0)
                             |   5|      WHILEM: matched 1 out of 0..1
   6 <I(2,3)> <.I.I>         |   6|       20:NOTHING(21)
   6 <I(2,3)> <.I.I>         |   6|       21:EXACT <.>(23)
   7 <I(2,3).> <I.I>         |   6|       23:WHILEM[1/3](0)
                             |   6|       WHILEM: matched 1 out of 0..32767
   7 <I(2,3).> <I.I>         |   7|        3:EXACT <I>(5)
   8 <I(2,3).I> <.I>         |   7|        5:CURLYX[0]{0,1}(20)
   8 <I(2,3).I> <.I>         |   8|         19:WHILEM(0)
                             |   8|         WHILEM: matched 0 out of 0..1
   8 <I(2,3).I> <.I>         |   9|          7:EXACT <(>(9)
                             |   9|          failed...
                             |   8|         WHILEM: failed, trying continuation...
   8 <I(2,3).I> <.I>         |   9|          20:NOTHING(21)
   8 <I(2,3).I> <.I>         |   9|          21:EXACT <.>(23)
   9 <I(2,3).I.> <I>         |   9|          23:WHILEM[1/3](0)
                             |   9|          WHILEM: matched 2 out of 0..32767
   9 <I(2,3).I.> <I>         |  10|           3:EXACT <I>(5)
  10 <I(2,3).I.I> <>         |  10|           5:CURLYX[0]{0,1}(20)
  10 <I(2,3).I.I> <>         |  11|            19:WHILEM(0)
                             |  11|            WHILEM: matched 0 out of 0..1
  10 <I(2,3).I.I> <>         |  12|             7:EXACT <(>(9)
                             |  12|             failed...
                             |  11|            WHILEM: failed, trying continuation...
  10 <I(2,3).I.I> <>         |  12|             20:NOTHING(21)
  10 <I(2,3).I.I> <>         |  12|             21:EXACT <.>(23)
                             |  12|             failed...
                             |  11|            failed...
                             |  10|           failed...
                             |   9|          WHILEM: failed, trying continuation...
   9 <I(2,3).I.> <I>         |  10|           24:NOTHING(25)
   9 <I(2,3).I.> <I>         |  10|           25:OPEN1(27)
   9 <I(2,3).I.> <I>         |  10|           27:EXACT <I>(29)
  10 <I(2,3).I.I> <>         |  10|           29:CLOSE1(31)
  10 <I(2,3).I.I> <>         |  10|           31:END(0)
Match successful!
Matched I.
Freeing REx: " (?:(?^x:I (?: \(\d (?:, \d)*\) )?) \.)* (I) "
Freeing REx: "I (?: \(\d (?:, \d)*\) )?"