Regex at Index == 0

[jehugaleahsa]

Hello:

I have an input string that is supposed to follow a particular syntax.
I am making a simple parser. In doing so, I need to write a lexical
analyzer. I have decided to implement it in terms of regular
expressions.

I want to take a list of regular expressions and iterate over them,
finding the first one that matches. Of course, it has to match AND the
match must start at index == 0.

The problem I have is that the regex has to look through the whole
string. I want to limit it to being the first position. Would
appending ^ to every regex work? I am just looking for a way to short-
circuit.

Thanks,
Travis

[Anthony Jones]

<je**********@gmail.comwrote in message
news:f1**********************************@f36g2000 hsa.googlegroups.com...

Hello:

I have an input string that is supposed to follow a particular syntax.
I am making a simple parser. In doing so, I need to write a lexical
analyzer. I have decided to implement it in terms of regular
expressions.

I want to take a list of regular expressions and iterate over them,
finding the first one that matches. Of course, it has to match AND the
match must start at index == 0.

The problem I have is that the regex has to look through the whole
string. I want to limit it to being the first position. Would
appending ^ to every regex work? I am just looking for a way to short-
circuit.

Yes ^ would require that the pattern match only if the first character in
the search string matches the first character in the expression.

How many RegExps are you using, if just a few had you considered using a
single RegExp:-

^(?:(exp1)|(expr2)|...|(exprN))

Also are you tokenising? E.g., a form of command and value list syntax?
--
Anthony Jones - MVP ASP/ASP.NET

[Martin Honnen]

je**********@gmail.com wrote:

I want to take a list of regular expressions and iterate over them,
finding the first one that matches. Of course, it has to match AND the
match must start at index == 0.

The problem I have is that the regex has to look through the whole
string. I want to limit it to being the first position. Would
appending ^ to every regex work? I am just looking for a way to short-
circuit.

If you want to ensure that your regular expression matches only from the
beginning of the string then you indeed need to prefix your regular
expression with ^.
--

Martin Honnen --- MVP XML
http://JavaScript.FAQTs.com/

[jehugaleahsa]

On Sep 23, 8:02*am, "Anthony Jones" <[e-mail]>
wrote:

<[e-mail] in message

news:f1**********************************@f36g2000 hsa.googlegroups.com...

Hello:

I have an input string that is supposed to follow a particular syntax.
I am making a simple parser. In doing so, I need to write a lexical
analyzer. I have decided to implement it in terms of regular
expressions.

I want to take a list of regular expressions and iterate over them,
finding the first one that matches. Of course, it has to match AND the
match must start at index == 0.

The problem I have is that the regex has to look through the whole
string. I want to limit it to being the first position. Would
appending ^ to every regex work? I am just looking for a way to short-
circuit.

Yes ^ would require that the pattern match only if the first character in
the search string matches the first character in the expression.

How many RegExps are you using, if just a few had you considered using a
single RegExp:-

^(?:(exp1)|(expr2)|...|(exprN))

Also are you tokenising? E.g., a form of command and value list syntax?

--
Anthony Jones - MVP ASP/ASP.NET- Hide quoted text -

- Show quoted text -

Yes, I am tokenizing. It is a syntax similar to a SQL where clause. It
seems simple enough. I have to know what type of token I have found. I
have that covered already with my token class. It tells me the token
type and the actual value. Hopefully this will work out smoothly.

[jehugaleahsa]

Yes, I am tokenizing. It is a syntax similar to a SQL where clause. It

seems simple enough. I have to know what type of token I have found. I
have that covered already with my token class. It tells me the token
type and the actual value. Hopefully this will work out smoothly.- Hide quoted text -

- Show quoted text -

Awesome! I got my parser working. The intent of the code was to allow
my database library to generate the WHERE clause of a SQL statement by
interpretting conditions on properties of my business objects.

So, say I have a Customer class with properties that map to a database
table. My library has features for mapping between database tables and
classes via XML or Attributes (similar to NHibernate or LINQ). I
wanted to allow the users of my library to get away from SQL
completely. So, they can now say something like this:

foreach (Customer customer in registry.CreateQuery<Customer>(
"(Name = {0} AND State = {1}) OR Type = {2}",
"Microsoft", "WA", "Preferred"))
{
}

My code will generate database-specific SQL with the actual table name
and column names used instead of the class and property names.

The cool thing now is that I don't need to write SQL anymore and so my
code becomes a little more database independent. I generate my classes
and attributes/mapping files using a tool I wrote too, so I don't have
to worry about writing that either.

DB development just became a whole lot easier for my users. Yay!

Thanks for everyone's input.

~Travis

[Mythran]

<je**********@gmail.comwrote in message
news:ed**********************************@i76g2000 hsf.googlegroups.com...

>Yes, I am tokenizing. It is a syntax similar to a SQL where clause. It
seems simple enough. I have to know what type of token I have found. I
have that covered already with my token class. It tells me the token
type and the actual value. Hopefully this will work out smoothly.- Hide
quoted text -

- Show quoted text -

Awesome! I got my parser working. The intent of the code was to allow
my database library to generate the WHERE clause of a SQL statement by
interpretting conditions on properties of my business objects.

So, say I have a Customer class with properties that map to a database
table. My library has features for mapping between database tables and
classes via XML or Attributes (similar to NHibernate or LINQ). I
wanted to allow the users of my library to get away from SQL
completely. So, they can now say something like this:

foreach (Customer customer in registry.CreateQuery<Customer>(
"(Name = {0} AND State = {1}) OR Type = {2}",
"Microsoft", "WA", "Preferred"))
{
}

My code will generate database-specific SQL with the actual table name
and column names used instead of the class and property names.

The cool thing now is that I don't need to write SQL anymore and so my
code becomes a little more database independent. I generate my classes
and attributes/mapping files using a tool I wrote too, so I don't have
to worry about writing that either.

DB development just became a whole lot easier for my users. Yay!

Thanks for everyone's input.

~Travis

Hmm....make sure you are protecting against injection attacks....

Mythran