3.6 Field Expressions

Field expressions (also known as “fexes”) are a way to select fields of a record. They also allow you to do certain transformations on the selected fields, such as changing their names.

A FEX comprises a sequence of elements separated by commas:

     ELEM_1,ELEM_2,...,ELEM_N

Each element makes a reference to one or more fields in a record identified by a given name and an optional subscript:

     Field_Name[min-max]

min and max are zero-based indexes. It is possible to refer to a field occupying a given position. For example, consider the following record:

     Name: Mr. Foo
     Email: foo@foo.com
     Email: foo@foo.org
     Email: mr.foo@foo.org

We would select all the emails of the record with:

     Email

The first email with:

     Email[0]

The third email with:

     Email[2]

The second and the third email with:

     Email[1-2]

And so on. It is possible to select the same field (or range of fields) more than once just by repeating them in a field expression. Thus, the field expression:

     Email[0],Name,Email

will print the first email, the name, and then all the email fields including the first one.

It is possible to include a rewrite rule in an element of a field expression, which specifies an alias for the selected fields:

     Field_Name[min-max]:Alias

For example, the following field expression specifies an alias for the fields named Email in a record:

     Name,Email:ElectronicMail

Since the rewrite rules only affect the fields selected in a single element of the field expression, it is possible to define different aliases to several fields having the same name but occupying different positions:

     Name,Email[0]:PrimaryEmail,Email[1]:SecondaryEmail

When that field expression is applied to the following record:

     Name: Mr. Foo
     Email: primary@email.com
     Email: secondary@email.com
     Email: other@email.com

the result will be:

     Name: Mr. Foo
     PrimaryEmail: primary@email.com
     SecondaryEmail: secondary@email.com
     Email: other@email.com

It is possible to use the dot notation in order to refer to field and sub-fields. This is mainly used in the context of joins, where new fields are created having compound names such as Foo_Bar. A reference to such a field can be done in the fex using dot notation as follows:

     Foo.Bar