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Non capturing group in regular expression and it's use

Recently I have been stuck in a regular expression where I had to use grouping in places, values of which were not required. It won't make sense to explain the problem without the text. So here it is


Controller Id Connection Status Connection State Secure Role
------------- ----------------- ---------------- ------ ------
1             Connected         Active           No     Slave
25            Disconnected      Idle             No     Master


My goal was to get the numeric values at the beginning of the line while still matching the whole line for safety (so that any other numeric values in the output don't match). As you see in the output, each column can contain a value from a list of fixed values. e.g. Connection Status can only contain Connected or Disconnected. That demands the following regular expression -




But with the above expression, we are fetching all the columns when all we want is the first column. This is where the non capturing group of regular expression comes …
Recent posts

Simple employee table with OpenStack Horizon DataTable module

I was working on OpenStack dashboard (aka Horizon) few past few months and I will share a simple way to build your data tables based on Horizon.
To give a brief introduction of Horizon, it is the official dashboard project of OpenStack, the cloud operating system. Horizon is a python Django application. It provides pre-defined modules for common web application requirements like tabs, workflow, table etc. From Grizzly release onward Horizon modules are separated from OpenStack specific code. It allows one to use Horizon modules independently.

To build a data table in Horizon you need to simply create a model class for your data. The important thing to remember here is that the class must have an "id" attribute. Horizon will build rows based on this id. It can be alpha numeric also. Though it is not mandatory to display the id column in the table, you can still have it. Below is the data model for my Employee table -

class Employee: """ Employee class r…

Get current UTC timestamp in Python

Converting current time to UTC (GMT) in python is a very common query that is searched in Google but, I did not find a compact answer when I did the search for my work. I figured out the code by some help from Google and some experimentation of my own. I'll share the code, in case it is useful to someone.

First I'll explain the Epoch time (or POSIX/Unix time) and UTC (Coordinated Universal Time) time format.
Epoch time is calculated as number of seconds elapsed since midnight Jan 1, 1970. It is used in all *nix systems and has become a standard when considering date/time from multiple systems. In python time module allows us to get the current epoch time in local time zone.


import time epoch_time_in_local_timezone = time.time()
UTC time is basically synonymous of GMT. It marks the 0 offset time zone. Time in all other time zones are calculated with positive/negative offset from UTC. It is used in many cases where there are multiple machines, located in different time zones a…

Upgrade wireshark version in Ubuntu

Wireshark is a great and may be most widely used network protocol analyzer. In last few months I had used it extensively in a network project. Wireshark was a great help in debugging hard to find protocol problems.

During this project I was using Ubuntu 10.04 as my development environment. I installed wireshark using
sudo apt-get install wireshark

But default wireshark version provided by Ubuntu 10.04 repository is 1.2.7 which was not suiting my purpose. I needed the newer versions of wireshark which provides some important features for analyzing protocols.

There are 2 ways to upgrade your wireshark version from the default version provided by Ubuntu -


Add a third party repository to your repo conf and you can upgrade the wireshark. But I do not prefer this way as it imposes the restriction of version that can be installed.Install wireshark from source. This is best way as you can install any version of wireshark in your Ubuntu.

Here are the steps to follow -
Download the wireshark sour…

Call an external program from MySQL trigger

MySQL is the choice of many when it comes to database. Its free and quite robust.
During one of our prototype implementations we had a requirement of calling some external processes when there is a change in a MySQL table. MySQL triggers are provided for the same purpose. They get executed when the table is changed in certain ways that is specified by the programmer. Now it is very easy (rather trivial) to do some thing in other MySQL database tables when trigger gets fired. But, our requirement was to call a C program.

Fortunately MySQL provides a way to implement your own functions, its called User Defined Functions (UDF). The "how to" is here. Now that we know, how to define your own functions and call them from MySQL events, we need to write our logic in a C program by following the interface provided by MySQL and we are done.

Wait a minute. That is already done by somebody. They have made a library of UDFs. One of them, LIB_MYSQLUDF_SYS does exactly what we want. It al…

Read a log file in tail mode from python

When you want to develop a log reader application, you'll need to keep reading the log. There is a Linux utility doing exactly the same. Its the tail command. tail blocks on the file till new entries are appended to it.

I needed the similar functionality in python and there was a small library providing it already. Its called filetail.py. The program performs tail functionality and also handles log rotation.

But in my case, the log file used to change (file name) depending on the date. filetail could not handle it as the file name has changed already. To handle this I have made following changes to the filetail code -

In nextline() method file size should be checked to be less or equal to the cursor position:


# maybe the log got rotated out from under us? if stat(self.path)[ST_SIZE] <= self.pos: # modified by sarbajit # file got truncated and/or re-created self._reset()
In _reset() method I have added the …

jQuery UI dialog without any close option

We use jQuery UI for designing web pages in out project. Some times back we had a scenario where the user log in to a dashboard was implemented using jQuery UI dialog (modal) component. Requirement was that the dashboard page should not be reached without passing the authentication process. But the dialog component of jQuery UI library provides a default X icon at the top left corner of the element which provides an option to close the dialog.

In our case, we wanted to reload the same page on click of the X icon. So that the user can not escape the login dialog by using the X icon. We achieved this by writing the page reload code inside dialog's close handler.  Everything worked fine until we had to handle the successful log in. Once user login succeeds we need to call dialog widget's close handler which reloads the page in turn. Because of this, even after a successful login, user gets the login dialog again.

The only way to stop this was, to remove the page reload code from…

Popular posts from this blog

Call an external program from MySQL trigger

MySQL is the choice of many when it comes to database. Its free and quite robust.
During one of our prototype implementations we had a requirement of calling some external processes when there is a change in a MySQL table. MySQL triggers are provided for the same purpose. They get executed when the table is changed in certain ways that is specified by the programmer. Now it is very easy (rather trivial) to do some thing in other MySQL database tables when trigger gets fired. But, our requirement was to call a C program.

Fortunately MySQL provides a way to implement your own functions, its called User Defined Functions (UDF). The "how to" is here. Now that we know, how to define your own functions and call them from MySQL events, we need to write our logic in a C program by following the interface provided by MySQL and we are done.

Wait a minute. That is already done by somebody. They have made a library of UDFs. One of them, LIB_MYSQLUDF_SYS does exactly what we want. It al…

Using Multi threading with Drools

Drools is a very popular rule engine. It is based on Rete algorithm. To understand what is a rule engine and how to use it check here. But here I will assume that you have some working knowledge on Drools.

What I am going to discuss in this post is 'how to use multi-threading with Drools'. Below is a snippet which creates a KnowledgeBase from a DRL file.



public void buildKnowledgeBase() { KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder(); kbuilder.add( ResourceFactory.newUrlResource( "file://myrules.drl" ), assertFalse( kbuilder.hasErrors() ); KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase(); }
If this method is called from multiple threads I get below exception -



Exception in thread "Thread-60" [Error: incomplete statement: (possible
use of reserved keyword as identifier: )]

[Near : {... globals != empt ....}]

^

[Line: 0, Column: 0]

at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterp…

Get JSON data response with JQuery Form plugin for a file upload form

My latest project involves JQuery for UI development. Frankly speaking, I did not have much idea of JQuery before. But I found it very easy to understand. All APIs are well documented with lots of examples available on Internet. The best feature of JQuery is its plugins. There are thousands of plugins available to play with. For every need of my web app I could find a plugin.



In my web app I am using JQuery Form plugin to submit the forms through AJAX. The return from my servlet is JSON object. The form plugin works great for all my forms. Well almost all, there is a HTML form which has a file upload box along with other input fields. This form was not able to get the response in JSON format. Though it worked in Google Chrome, but Firefox was presenting me with a file save dialog box.
I did a lot of googling to find out whats going on. Finally I observed that the problem is with HTTP headers that the form submit request has. Here are the headers captured by Chrome







As you can see the