1.2.3. Exists Matcher

The exists matcher has already been used to illustrate how a tag matcher or indicator matcher works. It can be used to verify the existence of a field. The expression consists of a tag matcher, an optional indicator matcher, followed by a question mark ? character.

The matcher evaluates to true if at least one field was found that matches the specified tag matcher and, if applicable, the indicator matcher.

Examples:

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>>
>>> df = read_marc21(filename, "001", predicate="024/7#?")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="0[45]5/*?")
>>> assert df.height == 0