Add and document path prefix as pattern style

The “extract” command supports extracting all files underneath a given
set of prefix paths. The forthcoming support for extracting files using
a pattern (i.e. only files ending in “.zip”) requires the introduction
of path prefixes as a third pattern style, making it also available for
exclusions.

borg/archiver.py

@@ -611,12 +611,12 @@ class Archiver:
 
     helptext = {}
     helptext['patterns'] = textwrap.dedent('''
-        Exclusion patterns support two separate styles, fnmatch and regular
-        expressions. If followed by a colon (':') the first two characters of
-        a pattern are used as a style selector. Explicit style selection is necessary
-        when regular expressions are desired or when the desired fnmatch pattern
-        starts with two alphanumeric characters followed by a colon (i.e.
-        `aa:something/*`).
+        Exclusion patterns support three separate styles, fnmatch, regular
+        expressions and path prefixes. If followed by a colon (':') the first two
+        characters of a pattern are used as a style selector. Explicit style
+        selection is necessary when a non-default style is desired or when the
+        desired pattern starts with two alphanumeric characters followed by a colon
+        (i.e. `aa:something/*`).
 
         `Fnmatch <https://docs.python.org/3/library/fnmatch.html>`_ patterns use
         a variant of shell pattern syntax, with '*' matching any number of
@@ -640,6 +640,10 @@ class Archiver:
         documentation for the re module
         <https://docs.python.org/3/library/re.html>`_.
 
+        Prefix path patterns can be selected with the prefix `pp:`. This pattern
+        style is useful to match whole sub-directories. The pattern `pp:/data/bar`
+        matches `/data/bar` and everything therein.
+
         Exclusions can be passed via the command line option `--exclude`. When used
         from within a shell the patterns should be quoted to protect them from
         expansion.

borg/helpers.py

@@ -412,6 +412,7 @@ class RegexPattern(PatternBase):
 
 _PATTERN_STYLES = set([
     FnmatchPattern,
+    PathPrefixPattern,
     RegexPattern,
 ])
 

borg/testsuite/helpers.py

@@ -324,6 +324,10 @@ def test_invalid_unicode_pattern(pattern):
      ["/more/data"]),
     ([r"  re:^\s  "], ["/data/something00.txt", "/more/data", "/home", "/whitespace/end\t"]),
     ([r"  re:\s$  "], ["/data/something00.txt", "/more/data", "/home", " #/wsfoobar", "\tstart/whitespace"]),
+    (["pp:./"], None),
+    (["pp:/"], [" #/wsfoobar", "\tstart/whitespace"]),
+    (["pp:aaabbb"], None),
+    (["pp:/data", "pp: #/", "pp:\tstart", "pp:/whitespace"], ["/more/data", "/home"]),
     ])
 def test_patterns_from_file(tmpdir, lines, expected):
     files = [
@@ -364,6 +368,12 @@ def test_patterns_from_file(tmpdir, lines, expected):
     ("re:.*", RegexPattern),
     ("re:^/something/", RegexPattern),
     ("re:re:^/something/", RegexPattern),
+
+    # Path prefix
+    ("pp:", PathPrefixPattern),
+    ("pp:/", PathPrefixPattern),
+    ("pp:/data/", PathPrefixPattern),
+    ("pp:pp:/data/", PathPrefixPattern),
     ])
 def test_parse_pattern(pattern, cls):
     assert isinstance(parse_pattern(pattern), cls)