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Diffstat (limited to 'doc/gawk.texi')
| -rw-r--r-- | doc/gawk.texi | 24 |
1 files changed, 22 insertions, 2 deletions
diff --git a/doc/gawk.texi b/doc/gawk.texi index 7d5aa4dc..84707907 100644 --- a/doc/gawk.texi +++ b/doc/gawk.texi @@ -6093,17 +6093,37 @@ regexp constants are valid and work the way you want them to, using any version of @command{awk}.@footnote{Use two backslashes if you're using a string constant with a regexp operator or function.} -Finally, when @samp{@{} and @samp{@}} appear in regexp constants +When @samp{@{} and @samp{@}} appear in regexp constants in a way that cannot be interpreted as an interval expression (such as @code{/q@{a@}/}), then they stand for themselves. As mentioned, interval expressions were not traditionally available in @command{awk}. In March of 2019, BWK @command{awk} (finally) acquired them. - Nonetheless, because they were not available for so many decades, @command{gawk} continues to not supply them when in compatibility mode (@pxref{Options}). +POSIX says that interval expressions containing repetition counts greater +than 255 produce unspecified results. + +@cindex Eggert, Paul +In the manual for GNU @command{grep}, Paul Eggert notes the following: + +@quotation +Interval expressions may be implemented internally via repetition. +For example, @samp{^(a|bc)@{2,4@}$} might be implemented as +@samp{^(a|bc)(a|bc)((a|bc)(a|bc)?)?$}. A large repetition count may +exhaust memory or greatly slow matching. Even small counts can cause +problems if cascaded; for example, @samp{grep -E +".*@{10,@}@{10,@}@{10,@}@{10,@}@{10,@}"} is likely to overflow a +stack. Fortunately, regular expressions like these are typically +artificial, and cascaded repetitions do not conform to POSIX so cannot +be used in portable programs anyway. +@end quotation + +@noindent +This same caveat applies to @command{gawk}. + @node Bracket Expressions @section Using Bracket Expressions @cindex bracket expressions |