2004.10_Txt2Regex-Generate Complex Regular Expressions the Easy Way.pdf

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txt2regex

Shell Wizard

For geeks, regular expressions might

be an interesting mental exercise, but

the jungle of arcane abbreviations

and symbols can be terrifying for

newbies. Enter the txt2regex editor.

BY ANDREAS KNEIB

expression to return a specific

string is a familiarly frustrating

experience for most readers who are not

regular expression gurus. As is the resig-

nation that slowly sets in when you

discover that your regex would work in

the Perl programming language, but not

in the Lisp code you just happen to be

working on.

Enter the regex wizard, txt2regex,

which we will be investigating in this

article. The wizard processes text pat-

terns for a variety of programs and

languages, from awk, through Emacs, to

Perl, procmail, sed, and vim. The

txt2regex program comprises mainly of a

shell script that requires a Bash version

2.04 or later.

Although nearly any state of the art

Linux distro should satisfy this require-

ment, you might just like to test this to

be on the safe side. Type echo

$BASH_VERSION , or bash --version to do

so. Assuming that your version number

is high enough, you can carry on with

the install.

If your distribution does not include

the wizard, check out the project down-

load page at [1] for

a tar archive.

Debian users can

simply enter apt-

get install txt2regex

to install the tool.

After completing

the download, first

unpack the archive

and change to the

directory created by doing so:

password)

root# make install

Figure 2: Displaying meta-characters.

However, there is a disadvantage to this

approach as it stores the program com-

ponents in the /usr/bin and

/usr/share/locale directo-

ries, where you really

should not install anything

foreign to your distribution.

To modify the target direc-

tories, pass the BINDIR and

LOCALEDIR variables to

make install . See Figure 1.

This puts the program

components in /usr/local/bin and /usr/

local/share/locale . There is another pos-

sible approach which involves editing

the variables in the Makefile txt2regex-

0.7/Makefile . The MANDIR variable is

not used in the version 0.7 Makefile. You

can copy txt2regex-0.7/txt2regex.man to

/usr/local/man/man1/ , for example, to

be able to call the manpages in future

( man txt2regex ):

~> tar xvzf U

txt2regex-0.7.tgz

~> cd txt2regex-0.7

As you do not need to

build the program, you

can simply enter: make

install as root :

Figure 1: Passing targets while making.

~> su

Password: (type root U

root# cp txt2regex.man /usr/ U

local/man/man1/txt2regex.1

October 2004

www.linux-magazine.com

Taking the headaches out of creating regular expressions

T rying to find the right regular

txt2regex

LINUX USER

Figure 3: Txt2regex – initial view.

Figure 4: Selecting programming languages.

The program is now ready to run. Type

txt2regex in a terminal window to launch

it.

--history is another important option,

as it loads a previously defined expres-

sion:

A question and answer session comes

next, helping you to define your regular

expression. The first set of data, start to

match: applies to where the search pat-

tern starts. This can be on the line

beginning or in any part of the line .

Answer by pressing 1 (for the beginning

of the line) or 2 (for anywhere). Your

input is converted to regex-speak and

appears in the regex boxes of the speci-

fied applications.

After finishing this selection, the next

group is entitled followed by: . It has a list

of nine numbered items, ranging from

any character , through a forbidden char-

acters list , to anything .

After setting the options for permitted

or forbidden characters, you are asked

which of these you want. The editor then

goes on to ask you how many times the

character should occur, offering you

seven options ranging from once to up to

N times.

Having completed this step, you are

again taken to the overview with the fol-

lowed by: heading. If you then select the

String option, you are prompted to type

the desired text string. Item 7, POSIX

combination , gives you access to a list in

alphabetical order which includes letters ,

numbers , hexadecimal numbers and

graphic chars . The dot key takes you

back out of the Posix menu.

The wizard uses this approach to com-

pile a text pattern for you, without you

needing to worry about the specifics of

special characters or the like. When you

have finished your regex, just press the

dot key.

Options

Let’s take a look at the tool’s command

line options first. There is a basic setting

for the background of the virtual text-

based console or Xterm. Output from

the wizard is more easily legible if

you opt for a light colored background;

the --whitebg parameter takes care of

this, as in the following example:

~>txt2regex --history '13ﬂ U

http://www.linux-magazine.com'

This displays the 13ﬂhttp://www.www.

linux-magazine.com string as a regex in

various programming languages. When

you complete an expression, the pro-

gram displays a shortcut for the history

option. You can type --showinfo for a list

of the program’s meta-characters. See

Figure 2.

The first line contains the program

name, egrep in our case, followed by

details on the meta-characters, the spe-

cial characters than need to be escaped,

or the Posix classes. For more informa-

tion on egrep , you can access the

manpages by typing man egrep .

~>txt2regex --whitebg

You can remove the color from txt2regex

by specifying --nocolor . You also need the

--prog option to specify the programs that

the script will display regex’s for. The

following example tells the tool to dis-

play the regular expressions for the vim

and emacs editors, as well as the Visual

Basic Script and Perl programming lan-

guages:

Question and Answer

After launching the wizard (see Figure 3)

by typing txt2regex in a console, you will

note the options at the top: quit , reset ,

color , and programs . Hitting the dot key

quits the editor, which then outputs a

regex expression that you can access via

the --history option. You are also shown

the text pattern that the regex specifies,

for example: Start match at beginning of

line, followed by a string…

When you press the 0 key to call the

reset option, the program resets all previ-

ous entries. In contrast to this, the

asterisk key, *, toggles the color on or

off. Press the / key to access Programs ,

the tool’s most important menu. This

will then take you to an overview screen

of the programs whose regular expres-

sions you want to use (see Figure 4), and

which txt2regex can help with.

~>txt2regex --prog vim,emacs, U

vbscript,perl

To display all possible regex variants, set

the --all option instead. You might note

that the program quits with a message

about your screen not having enough

lines, even though you are running it in a

large Xterm. You may want to export

COLUMNS LINES in Bash, before launch-

ing txt2regex --all to prevent this from

happening.

■

GLOSSARY

INFO

Regex: The abbreviation for regular expres-

sion; a combination of special characters that

programs can use to search for specific text

strings.

[1] Txt2regex:

http://txt2regex.sourceforge.net/

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October 2004

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