diff options
| -rw-r--r-- | ChangeLog | 5 | ||||
| -rw-r--r-- | txr.1 | 34 |
2 files changed, 29 insertions, 10 deletions
@@ -1,5 +1,10 @@ 2011-09-29 Kaz Kylheku <kaz@kylheku.com> + * txr.1: Clarified consecutive variables and documented double + variable match. + +2011-09-29 Kaz Kylheku <kaz@kylheku.com> + * parser.l: Implemented backslash continuations in SPECIAL state, regexes and string literals. @@ -580,16 +580,22 @@ useful for labeling information and situations. .SS Consecutive Variables -If an unbound variable is followed by another unbound variable, the -combination is a semantic error which will fail the query. A -diagnostic message will be issued, unless operating in quiet mode via -q. -The reason is that there is no way to bind two consecutive variables to -an extent of text; this is an ambiguous situation, since there is no -matching criterion for dividing the text between two variables. -(In theory, a repetition of the same variable, like @FOO@FOO, could -find a solution by dividing the match extent in half, which would work -only in the case when it contains an even number of characters. -This behavior seems to have dubious value). +If an unbound variable specified a fixed-width match or a regular expression, +then the issue of consecutive variables does not arise. Such a variable +consumes text regardless of any context which follows it. + +However, what if an unbound variable with no modifier is followed by another +variable? The behavior depends on the nature of the other variable. + +If the other variable also has no modifier, this is a semantic error which +will cause the query to fail. A diagnostic message will be issued, unless +operating in quiet mode via -q. The reason is that there is no way to bind two +consecutive variables to an extent of text; this is an ambiguous situation, +since there is no matching criterion for dividing the text between two +variables. (In theory, a repetition of the same variable, like @FOO@FOO, could +find a solution by dividing the match extent in half, which would work only in +the case when it contains an even number of characters. This behavior seems to +have dubious value). An unbound variable may be followed by one which is bound. The bound variable is replaced by the text which it denotes, and the logic proceeds @@ -612,6 +618,14 @@ If an unbound variable is followed by a variable which is bound to a list, or nested list, then each character string in the list is tried in turn to produce a match. The first match is taken. +An unbound variable may be followed by another unbound variable which specifies +a regular expression match. This is a special case called a "double variable +match". What happens is that the text is searched using the regular +expression. If the search fails, than neither variable is bound: it is a +matching failure. If the search succeeds, than the first variable is bound to +the text which is skipped by the regular expression search. The second +variable is bound to the text matched by the regular expression. + .SS Longest Match The closest-match behavior for text and regular expressions can be |