Evaluating minimum/maximum regex match length.

The application of regular expressions in data validations is a standard nowadays. A good regular expression in the right context may save the day, I believe no one can deny it. However, sometimes you (or other people using your software) need some more feedback if the provided data fails to match the test pattern. A typical situation may look like this:

def test_id(user_id):
    import re
    regex = r"(secret\s?)?agent\s([0-9]{3}|james bond)"
       
    if not re.match(regex, user_id):
        print "Incorrect user ID: %s" % user_id
    else: 
        print "OK: %s" % user_id

test_id('agent james bond')
test_id('secret agent 007')
test_id('agent jack black')
test_id('agent j')

Running this code results in the following output:

OK: agent james bond
OK: secret agent 007
Incorrect user ID: agent jack black
Incorrect user ID: agent j

So what if we want to give a hint, about the minimum and maximum length? In many cases guessing/manually evaluating the potential match size should be easy. However, if You plan using some more sophisticated regular expressions, You should not rely on Your intuition - but on calculations:

def test_id(user_id):
    import re
    import sre_parse

    regex = r"(secret\s?)?agent\s([0-9]{3}|james bond)"
    match_range = sre_parse.parse(regex).getwidth()

    if len(user_id) < match_range[0] or \
            len(user_id) > match_range[1]:
        print "User ID %s should be between %s and %s characters" % \
                (user_id, match_range[0], match_range[1])
    elif not re.match(regex, user_id):
        print "Incorrect user ID: %s" % user_id
    else: 
        print "OK: %s" % user_id



test_id('agent james bond')
test_id('secret agent 007')
test_id('agent jack black')
test_id('agent j')

This time the output should look like this:

OK: agent james bond
OK: secret agent 007
Incorrect user ID: agent jack black
User ID agent j should be between 9 and 23 characters

It certainly is more descriptive.
Cheers!

~KR

Regular expression based process termination in linux.

Even though Unix-based systems are generally stable, some processes (usually non-kernel) could benefit from a kill from time to time. GUI applications may hang, some batch programs may have memory leaks, there are many things that can go wrong. Fortunately Linux provides a set of tools that may help in such situations, you can use the ps command to list active processes. It reports a snapshot of a process granting you information about the process id (PID), parent process id (PPID), priority, memory usage, current state and more. There are also kill  and pkill commands which may terminate a process identified by a specific id or executable name respectively.

If you are running multiple instances of a program, and you want to terminate only a few of them its hard to apply the presented commands - pkill will terminate all instances (that is not desired), while kill will require you to obtain PIDs (this requires some work). It may be easier to locate the mentioned processes by their command line arguments, execution paths or other run parameters, literally or using regular expressions.

The following script does the job:

if [ "$1" ]
then
    ps lax  | tr -s ' ' | cut -d ' '  -f 3,13- | \
    egrep "$1" | cut -d ' ' -f 1 | \ 
    xargs kill -9 2>/dev/null
else
    echo "Usage: $0 <extended regular expresion>"
fi

First a process list is obtained and adjusted for further processing (the text output is so lousy...). Using cut preserves only the third column (PID) and everything beyond the 13-th (whitespace separated application name with additional parameters). Next we match the output with a provided extended regular expression (ERE), be warned though - the tested string starts with the process ID so starting the ERE with a "^" is a bad idea (starting with "^[0-9]+" may work, but you'll eventually end up with restarting your system :-)).

Cheers!

~KR