1.2.1. Tag Matcher

In MARC21, variable fields are uniquely identified by tags. A tag matcher is an expression used to filter those fields that match a specific tag.

In its simplest form, only the three numerical digits of a tag are specified. A match with a tag only exists if these digits exactly match:

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>> df = read_marc21(filename, "001", predicate="001?")
>>> assert df.height == 1

In order to identify more than one field, a pattern-based comparison must be performed. Each numerical digit of a tag can be specified by one of the following variants.

First, a digit can be represented by the wildcard character . that accepts all possible values from 0 to 9. For example, the following tag matcher 0.8 accepts all fields that that begins with 0 and ends with 8. The middle position can contain any digit.

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>> df = read_marc21(filename, "001", predicate="0.8?")
>>> assert df.height == 1

Next, a digit can also be represented by specifying a class of possible digits. In the following example 0[235]5, all fields that start with a 0, have either a 2, 3, or 5 in the second position, and end with a 5 are accepted.

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>> df = read_marc21(filename, "001", predicate="0[235]5?")
>>> assert df.height == 1

Similar to the character classes of a regular expression, several consecutive digits within a class can be combined into a range. The range is inclusive, and the upper interval limit must be greater than the lower limit. Note that a class can consist of more than one range expression.

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>> df = read_marc21(filename, "001", predicate="0[2-5]5?")
>>> assert df.height == 1

A class can also be specified in negated form (^). In this case, the matcher checks that the digit in the corresponding position of the tag does not originate from the class digits:

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>> df = read_marc21(filename, "001", predicate="04[^0-3]?")
>>> assert df.height == 0

After all, every options can be used in all positions of a tag matcher. In the most extreme case, the expressions ... and [0-9][0-9][0-9] accept every field, while the expression [^0-9][^0-9][^0-9] accepts no fields. In the following example, all fields that begin with any digit not followed by a 0 and end with a 1, 2 3, 5, or 6 are taken into account.

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>> df = read_marc21(filename, "001", predicate=".[^0][1-356]?")
>>> assert df.height == 1