From e48611ef36ee059b6e8791469f4cd7bbd610ba02 Mon Sep 17 00:00:00 2001
From: brxfork <ph.bricout@gmail.com>
Date: Thu, 23 Jun 2022 19:59:25 +0200
Subject: one-liner with regex for challenge 144

---
 challenge-144/brxfork/README       |  1 +
 challenge-144/brxfork/perl/ch-1.sh | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 challenge-144/brxfork/README
 create mode 100755 challenge-144/brxfork/perl/ch-1.sh

diff --git a/challenge-144/brxfork/README b/challenge-144/brxfork/README
new file mode 100644
index 0000000000..5eb20c6be7
--- /dev/null
+++ b/challenge-144/brxfork/README
@@ -0,0 +1 @@
+Solution by Philippe Bricout.
diff --git a/challenge-144/brxfork/perl/ch-1.sh b/challenge-144/brxfork/perl/ch-1.sh
new file mode 100755
index 0000000000..5c2d7672bf
--- /dev/null
+++ b/challenge-144/brxfork/perl/ch-1.sh
@@ -0,0 +1,32 @@
+#!/bin/sh
+perl -E 'while (++$n <= 100) {undef @pf ; $_="S" x $n ; m{^(S{2,}?)(??{ $1=~/^(S{2,})\1+$/?"(?!)":"" })\1+$(?{push @pf, length($1)})(?!)};next if (@pf<1 or @pf>2 or ($pf[0]*($pf[1]//$pf[0])!=$n)) ;print "$n is Semiprime as $n = $pf[0] x ",$pf[1]//$pf[0],"\n"}'
+#
+# Default variable $_ contains several "S" (exactly $n chars).  
+# 
+# 1) 
+# m{^(S{2,}?)(??{ $1=~/^(S{2,})\1+$/?"(?!)":"" })\1+$(?{push @pf, length($1)})(?!)}
+#   ---------                                    ---- 
+# 
+# This finds any factor of $n by capturing a sequence in $1 (factor == length($1) )
+#   for example it finds "SSS" in "SSSSSS" (3 is a factor of 6 :)
+#   
+# 2) 
+# m{^(S{2,}?)(??{ $1=~/^(S{2,})\1+$/?"(?!)":"" })\1+$(?{push @pf, length($1)})(?!)}
+#             ----------------------------------
+#        Here we verified the factor is a prime number 
+# (??{}) produces a pattern "(?!)" if $1 is not prime => always fail => backtracking
+# (??{}) produces a pattern "" if $1 is prime => always ok => continue 
+# 
+# 3)
+# m{^(S{2,}?)(??{ $1=~/^(S{2,})\1+$/?"(?!)":"" })\1+$(?{push @pf, length($1)})(?!)}
+#                                                    -------------------------
+#  When a prime factor is found, we keep it in @pf
+#
+# 4)
+# m{^(S{2,}?)(??{ $1=~/^(S{2,})\1+$/?"(?!)":"" })\1+$(?{push @pf, length($1)})(?!)}
+#                                                                             ----
+# This pattern always fails. Backtracking permits to find all prime factors.
+#
+# Result : we keep only numbers with exactly one or two prime factors
+# and we verify the product is correct (the square if only one prime factor).
+# So we don't keep 8 because 2*2!=8
-- 
cgit