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Partial Regular Expression Matching


I would like to identify partial matching of a regular expression, for a
stream of input, as described in the pcrepartial(3) manpage. Is this
possible with ruby Regexp, or would I have to wrap (a piece of) pcre?
(or implement my own regular expression engine, hah!)

As an aside, what I am really trying to do is write a lexer that works
on stream input, and can decide whether any of the eligible tokens match
before reading EOF (which may be a long, long way off both in bytes and
time). If you can think of another approach (that still uses regexes)
that'd work too.

Thanks

1. http://www.gammon.com.au/pcre/pcrepartial.html

On Tue, May 22, 2007 at 03:20:04PM +0900, Hans Fugal wrote:
> I would like to identify partial matching of a regular expression, for a
> stream of input, as described in the pcrepartial(3) manpage. Is this
> possible with ruby Regexp, or would I have to wrap (a piece of) pcre?
> (or implement my own regular expression engine, hah!)

It looks like someone has wrapped pcre already:
http://raa.ruby-lang.org/project/pcre/
but that's quite old so you might need to fiddle with it a bit.

> As an aside, what I am really trying to do is write a lexer that works
> on stream input, and can decide whether any of the eligible tokens match
> before reading EOF (which may be a long, long way off both in bytes and
> time). If you can think of another approach (that still uses regexes)
> that'd work too.

Well, you can use regexps to distinguish a complete token from a partial
one, simply by checking if it is followed by a character which is not part
of the token. A little care is needed to handle EOF correctly - at worst you
could just stick a sentinel character onto the end.

A simple example, which matches (\w+) and (\s+) as tokens:

require 'stringio'
stream = StringIO.new("wibble  bibble  boing")

token = ""
chunk = stream.read(1)
token << chunk if chunk
loop do
  case token
  when /\A\w+/
    match = $&
  when /\A\s+/
    match = $&
  else
    puts "Syntax error here! " + token.inspect
    break
  end

  if match.size < token.size or chunk.nil?
    puts "Match token: " + token.slice!(0,match.size).inspect
    break if chunk.nil?
  else
    #puts "Partial match: " + token.inspect
    chunk = stream.read(1)
    token << chunk if chunk
  end
end

This should also work if you use, say, read(4096) instead of read(1), so it
ought to be pretty efficient.

Regards,

Brian.

Well that works for \w+ an \s+, but what if you want to match /01+0/?
You'd get a syntax error on 0111 even though it's a valid partial match.
On Tue, May 22, 2007 at 03:20:04PM +0900, Hans Fugal wrote:
> I would like to identify partial matching of a regular expression, for a
> stream of input, as described in the pcrepartial(3) manpage. Is this
> possible with ruby Regexp, or would I have to wrap (a piece of) pcre?
> (or implement my own regular expression engine, hah!)

> As an aside, what I am really trying to do is write a lexer that works
> on stream input, and can decide whether any of the eligible tokens match
> before reading EOF (which may be a long, long way off both in bytes and
> time). If you can think of another approach (that still uses regexes)
> that'd work too.

Why not use rex?
Also StringScanner may be helpful for this.

On 5/22/07, Hans Fugal <fug@zianet.com> wrote:

> Well that works for \w+ an \s+, but what if you want to match /01+0/?
> You'd get a syntax error on 0111 even though it's a valid partial match.

Han's I'm not sure I understand your use case. Perhaps you could
provide some code as you would write it IF Ruby provided a
match_partial method for Regexp.

The normal use case for partial re matching is that you are processing
interactively accumulated input, and want to check that what the user
has typed in so far is a valid prefix for the valid inputs.  As far as
I can see the best way to do that with the current Ruby regexp engine
is to write a regexp which fully matches all prefixes

$ cat part_mat.rb
  full_pat = /^01+0/
  part_pat = /^((0|01+)0?)?$/

  (%w(0 01 010 0100 011 0110 01100) << "").each do |str|
    m = part_pat.match(str)
    if m
      puts "\"#{str}\" partially matches as \"#{m.string}\""
    else
      puts "\"#{str}\" does not match"
    end
  end

$ ruby part_mat.rb
"0" partially matches as "0"
"01" partially matches as "01"
"010" partially matches as "010"
"0100" does not match
"011" partially matches as "011"
"0110" partially matches as "0110"
"01100" does not match
"" partially matches as ""

It might be possible to take a regexp and automatically generate
another regexp which will match it's prefixes.

Might make an interesting rubyquiz.

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

On Wed, May 23, 2007 at 01:00:04AM +0900, Hans Fugal wrote:
> Well that works for \w+ an \s+, but what if you want to match /01+0/?
> You'd get a syntax error on 0111 even though it's a valid partial match.

OK, I see the problem - it's not detecting the end of the expression, it's
saying that this expression *might* match but only if the right characters
were appended to the end of the source.

In the general case I think you'd have to turn each RE into one which
matches all possible prefixes, perhaps something like

  /(0(1+(0)?)?)/   # (note *)

However, if you can guarantee that no individual valid token is going to be
longer than a certain size (let's say 200 characters) then it would be
simpler to ensure that you read-ahead at least 200 characters into a buffer
and then match against that.

Alternatively: perhaps only a few of your token REs have unlimited variable
length. Those you can code in the prefix form like that shown above. The
remainder (of fixed or limited length) can just be matched in the simple way
against a large enough read-ahead buffer.

Regards,

Brian.

(*) Hmm, this isn't quite right, since it partially matches 011112 as well.
You could check for a partial match (i.e. $3 = nil) and allow it only if it
consumes the whole string.

Alternatively, the RE itself needs to say "must be followed by X or end of
string". This works, but it's a bit ugly:

  /(0(\z|1+(\z|0)))

I can't think of a better formulation off the top of my head though.

Rick DeNatale wrote:
> On 5/22/07, Hans Fugal <fug@zianet.com> wrote:

>> Well that works for \w+ an \s+, but what if you want to match /01+0/?
>> You'd get a syntax error on 0111 even though it's a valid partial match.

> Han's I'm not sure I understand your use case. Perhaps you could
> provide some code as you would write it IF Ruby provided a
> match_partial method for Regexp.

It's a thought excercise. I've been fiddling with parser generators, and
had an idea for a simple recursive-descent parser that includes the
lexer by defining terminals as regexes. Example:

     # productions
     expr: term              {. expr0  = term  .}
         (   '+'  term       {. expr0 += term3 .}
           | '-'  term       {. expr0 -= term4 .}
         )*
         ;

     term: fact              {. term0  = fact1 .}
         (   '*' fact        {. term0 *= fact2 .}
           | '/' fact        {. term0 /= fact3 .}
         )*
         ;

     fact: ['+'] const       {. fact0 =  const1.to_f .}
         |  '-'  const       {. fact0 = -const2.to_f .}
         |  '(' expr ')'     {. fact0 =  expr1 .}
         ;

     # terminals
     const: /\d+[\.\d+]/ = '0';

Whether a parser generator or a generic lexer, in order to do the lexing
generically from a set of regexes, you need to be able to say "doesn't
match" in order to catch syntax errors in a timely manner. It's easy
enough if you have all of the input, or "a lot" which is reasonably
expected to be longer than any token, or if you can count on tokens not
crossing a guard (such as a newline), but in general you need to do
partial matching.

So the code might look like:

r = /\A#{terminals.inject {|u,r| Regexp.union(u,r)}}/
until input.eof?
   if r =~ input
     # figure out which token and consume/return it
   elsif r.partial_match(input)
     # wait for more input
   end
end

That may not be the most efficient way, but it gives a good idea.

The problem is also applicable for input verification, i.e. in a field
on a form, as has been mentioned.

I imagine it should be
const: /\d+(\.\d+)?/

> It's easy
> enough if you have all of the input, or "a lot" which is reasonably
> expected to be longer than any token, or if you can count on tokens not
> crossing a guard (such as a newline), but in general you need to do
> partial matching.

All your tokens above are one character, so a one character lookahead is
fine, apart from 'const'

If you read your input in 4096 byte blocks, reading a new block when your
buffer is less than 4096 bytes full, then you'll have somewhere between 4K
and 8K of lookahead. You'd do that for efficiency anyway, I'd hope.

So this leaves the case of any terminal regexps which might be required to
match more than 4K of data as a single token. If you're parsing a language
like that, then I'd agree that having partial matching makes your code a bit
simpler. But otherwise, you can write your regexp to match partially:

  const: /\d+(\.(\d+)?)?/

and if a partial match is detected, keep eating more input as necessary.

Note: the worst case is that the entire file consists of a single token - in
which case, you *will* end up reading the whole file into memory anyway.

Regards,

Brian.

Yes, of course. Sorry.

>> It's easy
>> enough if you have all of the input, or "a lot" which is reasonably
>> expected to be longer than any token, or if you can count on tokens not
>> crossing a guard (such as a newline), but in general you need to do
>> partial matching.

> All your tokens above are one character, so a one character lookahead is
> fine, apart from 'const'

My example is not meant to be pathological. It's not hard to imagine a
set of terminal regexes where you need much more than one-character
lookahead.

> If you read your input in 4096 byte blocks, reading a new block when your
> buffer is less than 4096 bytes full, then you'll have somewhere between 4K
> and 8K of lookahead. You'd do that for efficiency anyway, I'd hope.

When was the last time you typed 4k faster than the computer could
process it? The biggest reason for stream parsing is when there's a
human on the other end. In practice, you usually wouldn't have trouble
assuming newline is never part of a token, because you wouldn't get
stdin except after the user presses enter. But that's not entirely a
guarantee, and there can be other situations (like a network stream)
where that might not be the case.

> So this leaves the case of any terminal regexps which might be required to
> match more than 4K of data as a single token. If you're parsing a language
> like that, then I'd agree that having partial matching makes your code a bit
> simpler. But otherwise, you can write your regexp to match partially:

>   const: /\d+(\.(\d+)?)?/

> and if a partial match is detected, keep eating more input as necessary.

Easy enough in a hand-written lexer, but I wouldn't want to ask users of
a parser generator to have to provide partial-match regexes. And parsing
regexes to discover a partial match regex is not an easy task.

On Sun, May 27, 2007 at 12:45:12AM +0900, Hans Fugal wrote:
> >If you read your input in 4096 byte blocks, reading a new block when your
> >buffer is less than 4096 bytes full, then you'll have somewhere between 4K
> >and 8K of lookahead. You'd do that for efficiency anyway, I'd hope.

> When was the last time you typed 4k faster than the computer could
> process it? The biggest reason for stream parsing is when there's a
> human on the other end.

OK. So it sounds like the problem is that you *need* partial matching,
because you need to handle parse-as-you-type, rather than parse an existing
file presented as a stream.

In which case the conclusions seem clear:

(1) Find an existing regular expression engine or lexical analyser engine
which does what you need. There are several which have been ported to Ruby
which you could try.

or:

(2) Write your own Regular Expression engine.

Sorry for stating the obvious. But I can't really see what else you want
from this thread. Commiseration that Ruby's built-in regexp engine isn't
suitable for your requirements? :-)

Regards,

Brian.

Sorry that I didn't state the obvious. :) Yes, what I hoped to find from
this thread was that I wasn't missing something and that these are
really the only two options.
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