Regular Expression - Grouping Question

  • Thread starter Thread starter IKAS
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I

IKAS

I am having some difficulty in using Regular expression in .NET with
C#.

How do I write a regular expression so that it will match both the
following strings and also get me the relevant named groups ?

1. select xcol, ycol from sampletable where zcol='val'

2. select xcol, ycol from sampletable


If I try the following regular expression

^select (?<SelectClause>.*) from (?<FromClause>.*) (where
(?<WhereClause>.*))*$

and execute it with first string the "WhereClause" group is not
matched.
Even worse, the where clause part shows up in the "FromClause" group.

Maybe there is a better way to achieve what I am trying to do ?

Any help is appreciated.

Thanx in advance.
 
(e-mail address removed) (IKAS) wrote in
I am having some difficulty in using Regular expression in .NET
with C#.

How do I write a regular expression so that it will match both
the following strings and also get me the relevant named groups?

1. select xcol, ycol from sampletable where zcol='val'

2. select xcol, ycol from sampletable

If I try the following regular expression

^select (?<SelectClause>.*) from (?<FromClause>.*) (where
(?<WhereClause>.*))*$

and execute it with first string the "WhereClause" group is not
matched.
Even worse, the where clause part shows up in the "FromClause"
group.

Maybe there is a better way to achieve what I am trying to do ?
Click to expand...

The <FromClause> uses a greedy * quantifer. In this mode, * will
match as much text as it possibly can. Since the <WhereClause> is
optional, the <FromClause> * will "eat up" all of the text to the end
of the string.

To fix this, just put a ? after the * in <FromClause>:

^select (?<SelectClause>.*) from (?<FromClause>.*?)( where
(?<WhereClause>.*))*$

That turns the * quantifier into a non-greedy quantifier, and it will
only match the minimum amount of text necessary to fulfill its part
of the pattern.

Also note the change I made by putting the space before the "where"
keyword, because the space is really part of the optional
<WhereClause>.

Another change you might want to make is to use \s+ instead of hard-
coding the spaces to make the regex more flexible:

^select\s+(?<SelectClause>.*)\s+from\s+(?<FromClause>.*?)(\s+where\s+
(?<WhereClause>.*))*$


Hope this helps.

Chris.
 
Hi Chris !

Thanks a lot for your help.

I really appreciate for having taken the time to answer it.

I was actually trying some lazy constructs on the WhereClause...stupid
me...(;-))

Thanks,
IKAS
 
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