Pattern

The pattern specifies the structure of the filenames relative to the root directory. Parts that vary from file to file are indicated by groups, enclosed by parenthesis and preceded by ‘%’. They are represented by the Group class.

Each group definition starts with a name, and is then followed by multiple optional properties, separated by colons (in no particular order):

Property	Format	Description
Format string	:fmt=<format string>	Use a python format string to match this group in filenames.
Boolean format	:bool=<true>[:<false>]	Choose between two alternatives. The second option (false) can be omitted.
Custom regex	:rgx=<custom regex>	Specify a custom regular expression directly.
Optional flag	:opt	Mark the group as optional.
Discard flag	:discard	Discard the value parsed from this group when retrieving information.

So for instance, we can specify a filename pattern that will match an integer padded with zeros, followed by two possible options:

>>> "parameter_%(param:fmt=04d)_type_%(type:bool=foo:bar).txt"
parameter_0012_type_foo.txt
parameter_2020_type_bar.txt

Note

Groups are uniquely identified by their index in the pattern (starting at 0) and can share the same name. When using a name rather than an index, some functions may return more than one result if they are multiple groups with that name.

Warning

Groups are first found in the pattern by looking at matching parentheses. The pattern should thus have balanced parentheses or unexpected behavior can occur.

Name

The name of the group will dictate the regex and format string used for that group (unless overridden the ‘fmt’ and ‘rgx’ properties). The Group.DEFAULT_GROUPS class attribute will make the correspondence between name and regex:

Name		Regex	Format
F	Date (YYYY-MM-DD)	%Y-%m-%d	s
x	Date (YYYYMMDD)	%Y%m%d	08d
X	Time (HHMMSS)	%H%M%S	06d
Y	Year (YYYY)	\d{4}	04d
m	Month (MM)	\d\d	02d
d	Day of month (DD)	\d\d	02d
j	Day of year (DDD)	\d{3}	03d
B	Month name	[a-zA-Z]*	s
H	Hour 24 (HH)	\d\d	02d
M	Minute (MM)	\d\d	02d
S	Seconds (SS)	\d\d	02d
I	Index	\d+	d
text	Letters	\w	s
char	Character	\S*	s

Most of them are related to dates and follow the specification of strftime() and strptime() Behavior and strftime.

A letter preceded by a percent sign ‘%’ in the regex will be recursively replaced by the corresponding name in the table. This can be used in the custom regex. This still counts as a single group and its name will not be changed, only the regex. So %x will be replaced by %Y%m%d, in turn replaced by \d{4}\d\d\d\d. A percentage character in the regex is escaped by another percentage (‘%%’).

Format string

All the possible use cases are not covered in the table above. A simple way to specify a group is by using a format string following the Format Mini Language Specification. This will automatically be transformed into a regular expression.

Having a format specified has other benefits: it can be used to convert values into strings to generate a filename from parameters values (using Finder.make_filename), or vice-versa to parse filenames matches into parameters values.

It’s easy as scale_%(scale:fmt=.1f) which will find files such as scale_15.0 or scale_-5.6. Because we know how to transform a value into a string we can fix the group directly with a value:

finder.fix_group('scale', 15.)

or we can generate a filename:

>>> finder.make_filename(scale=2.5)
'scale_2.5'

In the opposite direction, we can retrieve a value from a filename:

>>> matches = finder.find_matches('scale_2.5')
>>> print(matches['scale'].get_match())
2.5  # a float

If the format is never specified, it defaults to a s format.

Warning

Only s, d, f, e, and E format types are supported.

Parsing of numbers will fail in some ambiguous (and quite unrealistic) cases that involves alignment padding with numbers or the minus signs. Creating a format object where we can’t unambiguously remove the padding character is not allowed and will raise a DangerousFormatError.

Similarly, for a string format (s) it can be impossible to separate correctly the alignment padding character (the “fill”) from the actual value. Here the user is entrusted with making sure the format fill character is adapted to the expected values to parse.

Boolean format

The boolean format allows to easily select between two strings. It is specified as :bool=<true>[:<false>]. The second option (false), can be omitted.

Here are a couple of examples. my_file%(special:bool=_special).txt would match both my_file.txt and my_file_special.txt. We could select only ‘special’ files using finder.fix_groups(special=True).

We can also specify both options with my_file_%(kind:bool=good:bad).txt, and select either like so

>>> finder.make_filename(kind=True)
my_file_good.txt
>>> finder.make_filename(kind=False)
my_file_bad.txt

Optional flag

The optional flag :opt marks the group as an optional part of the pattern. In effect, it appends a ? to the group regular expression. It does not affect the group in other ways.

Custom regex

Finally, one can directly use a regular expression. This will supersede the default regex, or the one generated from the format string if specified.

It can be done like so:

idx_%(idx:rgx=\d+?)

Discard keyword

Information can be retrieved from the matches in the filename, but one might discard a group so that it is not used. For example for a file of weekly averages with a filename indicating the start and end dates of the average, we might want to only recover the starting date:

sst_%(x)-%(x:discard)

Note

By default, when fixing a group to a value, discarded groups will not be fixed. This can be overridden with the fix_discard keyword argument.

Regex outside groups

By default, special characters (()[]{}?*+-|^$\\.&~# \t\n\r\v\f) outside of groups are escaped, and thus not interpreted as a regular expression. To use regular expressions outside of groups, it is necessary to pass use_regex=True when creating the Finder object.

Note

When using regex outside groups, Finder.make_filename won’t work.