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diff --git a/doc/gawktexi.in b/doc/gawktexi.in index 7b3c7ce8..e3b1bd2d 100644 --- a/doc/gawktexi.in +++ b/doc/gawktexi.in @@ -574,6 +574,7 @@ particular records in a file and perform operations upon them. * Fields with fixed data:: Field values with fixed-width data. * Splitting By Content:: Defining Fields By Content * More CSV:: More on CSV files. +* FS versus FPAT:: A subtle difference. * Testing field creation:: Checking how @command{gawk} is splitting records. * Multiple Line:: Reading multiline records. @@ -7913,6 +7914,7 @@ four, and @code{$4} has the value @code{"ddd"}. @menu * More CSV:: More on CSV files. +* FS versus FPAT:: A subtle difference. @end menu @c O'Reilly doesn't like it as a note the first thing in the section. @@ -8122,6 +8124,28 @@ a bed with a blanket that's not quite big enough. There's always a corner that isn't covered. We recommend, instead, that you use Manuel Collado's @uref{http://mcollado.z15.es/xgawk/, @code{CSVMODE} library for @command{gawk}}. +@node FS versus FPAT +@subsection @code{FS} Versus @code{FPAT}: A Subtle Difference + +As we discussed earlier, @code{FS} describes the data between fields (``what fields are not'') +and @code{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 @code{FS} or @code{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 @code{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 @code{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 @code{split()} +and @code{patsplit()} functions (@pxref{String Functions}). + @node Testing field creation @section Checking How @command{gawk} Is Splitting Records @@ -18505,6 +18529,16 @@ Nonalphabetic characters are left unchanged. For example, @code{toupper("MiXeD cAsE 123")} returns @code{"MIXED CASE 123"}. @end table +At first glance, the @code{split()} and @code{patsplit()} functions appear to be +mirror images of each other. But there are differences: + +@itemize @bullet +@item @code{split()} treats its third argument like @code{FS}, with all the +special rules involved for @code{FS}. + +@item Matching of null strings differs. This is discussed in @ref{FS versus FPAT}. +@end itemize + @sidebar Matching the Null String @cindex matching @subentry null strings @cindex null strings @subentry matching |