[TW-299] Make regular expressions behave more sed- or perl-like

_Vivenzio Pagliari on 2013-08-07T19:11:00Z says:_

# Current behaviour

Currently, _task_ has basic support for regular expressions in filtering and modifying task. This regex support is available setting:

(at)rc.regex = yes@

However, the current implementation is limited regarding substitution as compared to tools like _sed_ or _grep_, as it
does not support "capture groups". This is quite useful for "batch operations" on similar tasks.

Consider the following example:

$ for issue in \#1174 \#1315 ; do task add Fix $issue on DEVBRANCH ; done

$ for br in 2.x 2.4.0 ; do task '/^Fix #[0-9]+/' duplicate "/Fix (#[0-9]+) on DEVBRANCH/backport \1 to $br/" ; done

The output is not as expected:

$ task next


ID Age Urg  Description           

 1 3m     0 Fix #1174 on DEVBRANCH

 2 3m     0 Fix #1315 on DEVBRANCH

 3 2m     0 backport \1 to 2.x

 4 2m     0 backport \1 to 2.x    

 5 2m     0 backport \1 to 2.4.0

 6 2m     0 backport \1 to 2.4.0  

6 tasks

Note that a user accustomed to _sed_ behaviour would have likely written the last command (creation of backport tasks) as:

1.  Leave pattern in subsitution empty, this would be repetitive; like in: $ sed '/foo/ s//bar/'

$ for br in 2.x 2.4.0 ; do task '/^Fix (#[0-9]+) on DEVBRANCH/' duplicate "//backport \1 to $br/" ; done

Such a thing is possible in _sed_ (and I think in _perl_ as well) as there the regex match from the "filter" (address in sed-speak)
is remembered and need not be repeated in the pattern to substitute. If the to-be-substituted pattern is empty, the match from the
address is used.

However, this command produces a totally unexpected results:

$ task

[task next]


ID Age Urg  Description                          

 7 2s   0.9 Fix #1174 on DEVBRANCH

              8/7/2013 / /backport \1 to 2.x/

              8/7/2013 / /backport \1 to 2.4.0/

 8 2s   0.9 Fix #1315 on DEVBRANCH               

              8/7/2013 / /backport \1 to 2.x/    

              8/7/2013 / /backport \1 to 2.4.0/  

 3 5s   0.8 Fix #1174 on DEVBRANCH

              8/7/2013 / /backport \1 to 2.x/

 4 5s   0.8 Fix #1315 on DEVBRANCH               

              8/7/2013 / /backport \1 to 2.x/    

 5 2s   0.8 Fix #1174 on DEVBRANCH

              8/7/2013 / /backport \1 to 2.4.0/

 6 2s   0.8 Fix #1315 on DEVBRANCH               

              8/7/2013 / /backport \1 to 2.4.0/  

 1 14s    0 Fix #1174 on DEVBRANCH

 2 14s    0 Fix #1315 on DEVBRANCH               

8 tasks

# Requested modification
1. Support capture groups correctly, so that they can be back-referenced in substitutions.
1. When the substitution command has an empty pattern and the filter contains a regex, let that match be used.
# Additional notes

The feature request [[#1174]] asks for sed-like substitution command, which might be seen as related.