matcher: fix issues regex flag contained in pattern (issue6759)

Python 3.11 is now enforcing that flag must be at the beginning of the regex

This creates a serious regression for people using Python 3.11 with an hgignore
using flag in a "relre" pattern.

We now detect any flags in such pattern and "prepend" our ".*" pattern after them.

In addition, we now insert the flag in the regexp to only affect the pattern we
are rewriting. Otherwise, the regex built from the combined pattern would these
flags in the middle of it anyway.

As a side effect of this last change, we fix a bug… before this change regex
flag in a pattern would affect all combined patterns. That was bad and is not
longer the case.

The Rust code needs to be updated to fix that very bug, but we will do it in
another changeset.

mercurial/match.py

@@ -1323,6 +1323,9 def _globre(pat):
     return res
 
 
+FLAG_RE = util.re.compile(b'^\(\?([aiLmsux]+)\)')
+
+
 def _regex(kind, pat, globsuffix):
     """Convert a (normalized) pattern of any kind into a
     regular expression.
@@ -1353,6 +1356,8 def _regex(kind, pat, globsuffix):
     if kind == b'relre':
         if pat.startswith(b'^'):
             return pat
+        if FLAG_RE.match(pat):
+            return FLAG_RE.sub(br'(?\1:.*', pat) + b')'
         return b'.*' + pat
     if kind in (b'glob', b'rootglob'):
         return _globre(pat) + globsuffix

tests/test-hgignore.t

@@ -63,6 +63,50 Should display baz only:
   abort: $TESTTMP/ignorerepo/.hgignore: invalid pattern (relre): *.o (glob)
   [255]
 
+Test relre with flags (issue6759)
+---------------------------------
+
+regexp with flag is the first one
+
+  $ echo 're:(?i)\.O$' > .hgignore
+  $ echo 're:.hgignore' >> .hgignore
+  $ hg status
+  A dir/b.o
+  ? a.c
+  ? syntax
+
+regex with flag is not the first one
+
+  $ echo 're:.hgignore' > .hgignore
+  $ echo 're:(?i)\.O$' >> .hgignore
+  $ hg status
+  A dir/b.o
+  ? a.c
+  ? syntax
+
+flag in a pattern should affect that pattern only
+
+  $ echo 're:(?i)\.O$' > .hgignore
+  $ echo 're:.HGIGNORE' >> .hgignore
+  $ hg status
+  A dir/b.o
+  ? .hgignore (no-rust !)
+  ? .hgignore (rust missing-correct-output !)
+  ? a.c
+  ? syntax
+
+  $ echo 're:.HGIGNORE' > .hgignore
+  $ echo 're:(?i)\.O$' >> .hgignore
+  $ hg status
+  A dir/b.o
+  ? .hgignore
+  ? a.c
+  ? syntax
+
+
+further testing
+---------------
+
   $ echo 're:^(?!a).*\.o$' > .hgignore
   $ hg status
   A dir/b.o