attempt to allow look-behind assertions in tokens

instead of consuming matched text, offset is increased
and regex matching is tested from offset.

Note: since we remove the '^' anchor in the preg_match test
this can lead to degraded performance.

this allows for example following pp :

```
%skip       SPACE       \s
%token      FOO         foo(?= bar)
%token      BAR         bar
%token      BAZ         (?<=bar )baz

#doc:
    <FOO> <BAR> <BAZ>
```

with following string :

```
foo bar baz
```

----
maybe this can help #5