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4.7.2 FS Versus FPAT: A Subtle Difference

As we discussed earlier, FS describes the data between fields (“what fields are not”) and FPAT describes the fields themselves (“what fields are”). This leads to a subtle difference in how fields are found when using regexps as the value for FS or FPAT.

In order to distinguish one field from another, there must be a non-empty separator between each field. This makes intuitive sense—otherwise one could not distinguish fields from separators.

Thus, regular expression matching as done when splitting fields with FS is not allowed to match the null string; it must always match at least one character, in order to be able to proceed through the entire record.

On the other hand, regular expression matching with FPAT can match the null string, and the non-matching intervening characters function as the separators.

This same difference is reflected in how matching is done with the split() and patsplit() functions (see String-Manipulation Functions).