Pratt-parser
in [General Programming]
|
From: Ben Bacarisse on 6 Nov 2009 07:13 James Dow Allen <jdallen2000(a)yahoo.com> writes: > On Nov 6, 1:36 am, Ben Pfaff <b...(a)cs.stanford.edu> wrote: >> I looked around the web for a while and found a few superficial >> descriptions of SNOBOL patterns.... >> >> Can you give a few examples? > > Another nice feature of the SNOBOL programming language > is that the string matching a subpattern can be saved in > a variable. IIRC, > A B.X C You recall the operator correctly but, unless it is my turn to misremember, SNOBOL is very fussy about spaces and you must write: A B . X C > is the same as > A B C > but the string matching the subpattern B is saved in X. In case anyone is puzzled, SNOBOL has 2 of these assignment operators. The . assigns on a successful match and $ (that I illustrated elsewhere) does so, repeatedly, during the match. > I've often wanted to use this feature when sed'ing, e.g. > sed "sqhas [0-9]* wivesqwill have N happy widowsq" > where "N" is the matching [0-9]* remembered. Which, of course, you can do. Using a name is nicer, but \1 does the job. In SNOBOL, you use pretty much anything as the replacement: LINE SPAN('0123456789') . N = N + 1 increments the first number found in the variable LINE. -- Ben.
From: Christian Gollwitzer on 6 Nov 2009 10:24 James Dow Allen schrieb: > I've often wanted to use this feature when sed'ing, e.g. > sed "sqhas [0-9]* wivesqwill have N happy widowsq" > where "N" is the matching [0-9]* remembered. Use backreferences: $ sed 'sqhas \([0-9]*\) wivesqwill have \1 happy widowsq' has 9 wives will have 9 happy widows has 4 wives will have 4 happy widows Christian
From: Ben Pfaff on 6 Nov 2009 12:20 Ben Bacarisse <ben.usenet(a)bsb.me.uk> writes: > Ben Pfaff <blp(a)cs.stanford.edu> writes: >> I looked around the web for a while and found a few superficial >> descriptions of SNOBOL patterns. I also found a few SNOBOL >> reference manuals, but again their descriptions of patterns were >> very brief and I found it difficult to figure out how they were >> used and why exactly they were so powerful. >> >> Can you give a few examples? > > The main feature is that patterns are built from pattern primitives > using operators. Patterns are named so you can write: Thank you for the examples. -- "Long noun chains don't automatically imply security." --Bruce Schneier
From: James Dow Allen on 7 Nov 2009 02:53
On Nov 6, 10:24 pm, Christian Gollwitzer <Christian.Gollwit...(a)uni- bayreuth.de> wrote: > James Dow Allen schrieb: > > > I've often wanted to use this feature when sed'ing, e.g. > > sed "sqhas [0-9]* wivesqwill have N happy widowsq" > > where "N" is the matching [0-9]* remembered. > > Use backreferences: > > $ sed 'sqhas \([0-9]*\) wivesqwill have \1 happy widowsq' > has 9 wives > will have 9 happy widows > has 4 wives > will have 4 happy widows > > Christian Thanks! I had a hunch sed had such a facility and thought it easier to ask here than read the man. :-) Odd that I remembered a feature (forgetting the spaces) of a language (SNOBOL) which I've not used for *40 years* ... yet can't remember what I had for breakfast yesterday. James |