util: teach stringmatcher to handle forced case insensitive matches

The 'author' and 'desc' revsets are documented to be case insensitive.
Unfortunately, this was implemented in 'author' by forcing the input to
lowercase, including for regex like '\B'.  (This actually inverts the meaning of
the sequence.)  For backward compatibility, we will keep that a case insensitive
regex, but by using matcher options instead of brute force.

This doesn't preclude future hypothetical 'icase-literal:' style prefixes that
can be provided by the user.  Such user specified cases can probably be handled
up front by stripping 'icase-', setting the variable, and letting it drop
through the existing code.

mercurial/util.py

@@ -1996,7 +1996,7 b' def matchdate(date):'
         start, stop = lower(date), upper(date)
         return lambda x: x >= start and x <= stop
 
-def stringmatcher(pattern):
+def stringmatcher(pattern, casesensitive=True):
     """
     accepts a string, possibly starting with 're:' or 'literal:' prefix.
     returns the matcher name, pattern, and matcher function.
@@ -2006,6 +2006,9 b' def stringmatcher(pattern):'
     >>> def test(pattern, *tests):
     ...     kind, pattern, matcher = stringmatcher(pattern)
     ...     return (kind, pattern, [bool(matcher(t)) for t in tests])
+    >>> def itest(pattern, *tests):
+    ...     kind, pattern, matcher = stringmatcher(pattern, casesensitive=False)
+    ...     return (kind, pattern, [bool(matcher(t)) for t in tests])
 
     exact matching (no prefix):
     >>> test('abcdefg', 'abc', 'def', 'abcdefg')
@@ -2022,18 +2025,35 b' def stringmatcher(pattern):'
     unknown prefixes are ignored and treated as literals
     >>> test('foo:bar', 'foo', 'bar', 'foo:bar')
     ('literal', 'foo:bar', [False, False, True])
+
+    case insensitive regex matches
+    >>> itest('re:A.+b', 'nomatch', 'fooadef', 'fooadefBar')
+    ('re', 'A.+b', [False, False, True])
+
+    case insensitive literal matches
+    >>> itest('ABCDEFG', 'abc', 'def', 'abcdefg')
+    ('literal', 'ABCDEFG', [False, False, True])
     """
     if pattern.startswith('re:'):
         pattern = pattern[3:]
         try:
-            regex = remod.compile(pattern)
+            flags = 0
+            if not casesensitive:
+                flags = remod.I
+            regex = remod.compile(pattern, flags)
         except remod.error as e:
             raise error.ParseError(_('invalid regular expression: %s')
                                    % e)
         return 're', pattern, regex.search
     elif pattern.startswith('literal:'):
         pattern = pattern[8:]
-    return 'literal', pattern, pattern.__eq__
+
+    match = pattern.__eq__
+
+    if not casesensitive:
+        ipat = encoding.lower(pattern)
+        match = lambda s: ipat == encoding.lower(s)
+    return 'literal', pattern, match
 
 def shortuser(user):
     """Return a short representation of a user name or email address."""