Bug 30233: (follow-up) Add tests for _apply_substitution and combined modifiers

- t/C4/ClassSplitRoutine/RegEx.t (new): exercises _apply_substitution via
  the public split_callnumber interface — basic substitution, alternate
  delimiters, \n/$n backrefs, /gi combined flag, /e rejection, and malformed
  or uncompilable rules (including the new logger warn path).
- t/Koha/Regex/Replacement.t: extend the subst() helper to handle the ig/gi
  combined modifier and add a subtest that confirms case-insensitive global
  replacement and backref case preservation.

Signed-off-by: Martin Renvoize <martin.renvoize@openfifth.co.uk>
Signed-off-by: Baptiste Wojtkowski <baptiste.wojtkowski@biblibre.com>

t/C4/ClassSplitRoutine/RegEx.t

@@ -0,0 +1,76 @@
+#!/usr/bin/perl
+
+# This file is part of Koha.
+#
+# Koha is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Koha is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Koha; if not, see <https://www.gnu.org/licenses>.
+
+use Modern::Perl;
+
+use Test::NoWarnings;
+use Test::More tests => 6;
+
+use C4::ClassSplitRoutine::RegEx;
+
+# Exercise _apply_substitution through the public split_callnumber interface.
+# split_callnumber($string, \@regexes) applies each regex in turn and splits
+# on newlines; a single-rule list with no \n in the result returns a
+# one-element list.
+sub apply {
+    my ( $string, $regex ) = @_;
+    my @parts = C4::ClassSplitRoutine::RegEx::split_callnumber( $string, [$regex] );
+    return $parts[0] // $string;
+}
+
+subtest 'basic substitution' => sub {
+    plan tests => 3;
+    is( apply( 'foo', 's/foo/bar/' ),  'bar', 'simple replacement' );
+    is( apply( 'foo', 's/foo/bar/g' ), 'bar', '/g flag' );
+    is( apply( 'FOO', 's/foo/bar/i' ), 'bar', '/i flag' );
+};
+
+subtest 'alternate delimiters' => sub {
+    plan tests => 3;
+    is( apply( 'foo', 's#foo#bar#' ), 'bar', 'hash delimiter' );
+    is( apply( 'foo', 's|foo|bar|' ), 'bar', 'pipe delimiter' );
+    is( apply( 'foo', 's!foo!bar!' ), 'bar', 'bang delimiter' );
+};
+
+subtest 'backreference and flag combinations' => sub {
+    plan tests => 3;
+    is( apply( '20260623', 's/(\d{4})(\d{2})(\d{2})/$1-$2-$3/' ), '2026-06-23', '$n backrefs' );
+    is( apply( '20260623', 's/(\d{4})(\d{2})(\d{2})/\1-\2-\3/' ), '2026-06-23', '\n backrefs' );
+    is( apply( 'FOO foo',  's/foo/bar/gi' ),                      'bar bar',    '/gi combined' );
+};
+
+subtest '/e flag is rejected (no code execution)' => sub {
+    plan tests => 3;
+    is( apply( 'foo', 's/foo/1+1/e' ),   'foo', '/e alone: string unchanged' );
+    is( apply( 'foo', 's/foo/1+1/ge' ),  'foo', '/ge: string unchanged' );
+    is( apply( 'foo', 's/foo/1+1/ige' ), 'foo', '/ige: string unchanged' );
+};
+
+subtest 'malformed and uncompilable rules are silently ignored' => sub {
+    plan tests => 3;
+
+    # Not an s/// at all — ignored
+    is( apply( 'foo', 'm/foo/' ), 'foo', 'non-substitution regex ignored' );
+
+    # Missing closing delimiter
+    is( apply( 'foo', 's/foo/bar' ), 'foo', 'truncated s/// ignored' );
+
+    # Invalid pattern — compilation fails (also exercises the logger warn path)
+    my @warnings;
+    local $SIG{__WARN__} = sub { push @warnings, @_ };
+    is( apply( 'foo', 's/[invalid/bar/' ), 'foo', 'bad pattern ignored' );
+};

t/Koha/Regex/Replacement.t

@@ -18,7 +18,7 @@
 use Modern::Perl;
 
 use Test::NoWarnings;
-use Test::More tests => 5;
+use Test::More tests => 6;
 use File::Temp qw( tempdir );
 use File::Spec;
 
@@ -31,7 +31,8 @@ sub subst {
     my ( $value, $search, $replacement, $modifiers ) = @_;
     $modifiers //= '';
     my $expand = sub { Koha::Regex::Replacement::expand_template( $replacement, [ @{^CAPTURE} ], {%+} ) };
-    if    ( $modifiers eq 'g' ) { $value =~ s/$search/$expand->()/ge }
+    if    ( $modifiers =~ /g/ && $modifiers =~ /i/ ) { $value =~ s/$search/$expand->()/ige }
+    elsif ( $modifiers eq 'g' ) { $value =~ s/$search/$expand->()/ge }
     elsif ( $modifiers eq 'i' ) { $value =~ s/$search/$expand->()/ie }
     else                        { $value =~ s/$search/$expand->()/e }
     return $value;
@@ -79,6 +80,17 @@ subtest 'legacy \1 backreferences and fixed escapes' => sub {
     is( subst( 'x', '(x)', '\$1' ),  '$1',   '\$ emits a literal dollar, not a backref' );
 };
 
+subtest 'combined ig/gi modifiers' => sub {
+    plan tests => 3;
+    is( subst( 'FOO foo', 'foo', 'bar', 'ig' ), 'bar bar', '/ig: case-insensitive and global' );
+    is( subst( 'FOO foo', 'foo', 'bar', 'gi' ), 'bar bar', '/gi: same with reversed flag order' );
+    is(
+        subst( 'FOO foo', '(foo)', '[$1]', 'ig' ),
+        '[FOO] [foo]',
+        '/ig: backrefs preserve original case per match'
+    );
+};
+
 subtest 'malicious replacements are inert (no code execution)' => sub {
     plan tests => 4;