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 those of the tag:

$ marc21 count tests/data/ada.mrc --where '001 == "119232022"'
1

$ marc21 count tests/data/ada.mrc --where '002 == "xyz"'
0

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 accepts all fields that contains at least one field that begins with 0 and ends with 8. The middle position can contain any digit.

$ marc21 count tests/data/ada.mrc --where '0.8?'
1

Furthermore, a digit can also be represented by specifying a class of possible digits. In the following example, all fields that start with a zero, have either a two, three, or five in the second position, and end with a 5 are accepted.

$ marc21 count tests/data/ada.mrc --where '0[235]5?'
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.

$ marc21 count tests/data/ada.mrc --where '0[2-5]5?'
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:

$ marc21 count tests/data/ada.mrc --where '04[^0-3]?'
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 0and end with a 1, 2, 3, 5, or 6 are taken into account.

$ marc21 count tests/data/ada.mrc --where '.[^0][1-356].2 == "sswd"'
1