Re: question: text substitution using Perl
On 10/23/06, Steven W. Orr [EMAIL PROTECTED] wrote:
The difference between single quotes and double quotes is strictly
whether variable interpolation occurs.
Most importantly but not strictly true. The other difference between '
and is in escape sequence quoting. Only \ and ' are quoted by \
inside 's.
Note that q{} and qq{} are aliases for ' ' and , and you can use
your choice of bracketing characters.
http://perldoc.perl.org/perlop.html#Regexp-Quote-Like-Operators
--
Bill
[EMAIL PROTECTED] [EMAIL PROTECTED]
Boston Perl Mongers, http://boston.pm.org 2nd Tuesdays @ MIT
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Re: question: text substitution using Perl
Dear Kevin,
Thanks a lot for your help. I have 3 derived questions from your script.
Question 1:
For the back references, you used ${1} for the 1st captured buffer,
while some books/people simply use $1, I'm wondering if you use {} as an
extra caution to make sure it refers to the 1st captured buffer in case
there are some digits followed.
Question 2:
line 1 perl -i.bak \
line 2-pe 's/ \$(\d+)\. / \$ebcdic${1}. /g;
line 3 s/ (\d+)\. / s370ff${1}. /g;' \
line 4your-directory-somewhere/*readme*
On the end of line 1 and 3, you have a back slash. Is it for separating
input to separated lines for better readability? If so, why is there no
back slash on the end of line 2?
Question 3:
You used a period . after ${1}, wouldn't it be safe to use \. as the
original string only ends with a period and we don't want to change it?
I think . can match any single character except a newline.
Thank you for your time.
Zhao
Kevin D. Clark wrote:
Zhao Peng writes:
substitution 1
The characteristic of original string:
1, always start with $
2, then followed by an integer, could be more than 1 digit, such as 23
3, always end with a period .
4, there is always a blank before after original string
For example: $2.
The characteristic of target string: always has ebcdic inserted into
the original string between $ and the integer
For example: $ebcdic2.
So the substitution will look like this
$2. - $ebcdic2.
$67. - $ebcdic67.
Should the regular expression for original string be: \$\d+\. ?
Looks pretty much right to me.
I would make this replacement like this:
s/ \$(\d+)\. / \$ebcdic${1}. /g;
The ${1} in there is something called a backreference.
The 'g' at the end of the line generally specifies do this as many
times as possible on each line.
substitution 2
The characteristic of original string:
1, always start with an integer, could be more than 1 digit, such as 23
2, then end with a period .
3, there is always a blank before after original string
For example: 2.
The characteristic of target string:
always has s370ff added to the beginning of original string
For example: s370ff2.
So the substitution will look like this
2. - s370ff2.
14. - s370ff14.
Should the regular expression for original string be: \d+\. ?
I would make this replacement like this:
s/ (\d+)\. / s370ff${1}. /g;
My real situation is that I have a bunch of files at one directory, of
which for the files whose name contained readme, I need to do 2
substitutions described above.
One way to quickly do this might be like this:
perl -i.bak \
-pe 's/ \$(\d+)\. / \$ebcdic${1}. /g;
s/ (\d+)\. / s370ff${1}. /g;' \
your-directory-somewhere/*readme*
This in itself makes a backup for you, but you might want to make your
own backup files beforehand.
Just another Perl hacker,
--kevin
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Re: question: text substitution using Perl
Zhao Peng writes:
Thanks a lot for your help. I have 3 derived questions from your script.
No problem, glad to help.
Question 1:
For the back references, you used ${1} for the 1st captured buffer,
while some books/people simply use $1, I'm wondering if you use {} as
an extra caution to make sure it refers to the 1st captured buffer in
case there are some digits followed.
No, I wrote it this way just to make the code clear -- there wasn't a
technical reason for this.
Question 2:
line 1 perl -i.bak \
line 2-pe 's/ \$(\d+)\. / \$ebcdic${1}. /g;
line 3 s/ (\d+)\. / s370ff${1}. /g;' \
line 4your-directory-somewhere/*readme*
On the end of line 1 and 3, you have a back slash. Is it for
separating input to separated lines for better readability? If so, why
is there no back slash on the end of line 2?
There is no backslash on line 2 because the shell knows that it is
reading a literal string and that the end of the string is marked by
the trailing single-quote (') character.
Just to be absolutely clear, this has nothing to do with Perl. This
notation has everything to do with the shell. If you're using Linux,
your shell might be something like bash or csh or zsh.
To see what is going on here you might try typing
echo hello there
echo hello \
there
echo 'hello\nthere'
echo hello\nthere
echo $PATH
echo $PATH
echo '$PATH'
...into your shell (especially the first two examples).
Question 3:
You used a period . after ${1}, wouldn't it be safe to use \. as
the original string only ends with a period and we don't want to
change it? I think . can match any single character except a newline.
No...it wouldn't be safer to use \. instead of . in the
replacement part of the expression because the notation of . only
has the semantics of (generally) match any character except newline
in the *match* part of the expression, *not* in the replacement part
of the expression.
Perhaps this example helps illustrate this?
echo 2a | perl -pe 's/\d./hello./'
That last . is interpreted fairly literally in this context.
Regards,
--kevin
--
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Re: question: text substitution using Perl
Mike and Kevin, Thank you both for quick, detailed, and crystal clarification and explanation. Just one more question: To the shell, anything inside of single quotes is a single argument, so all of the spaces, newlines, etc. in there are passed without shell interpretation (quoted from the Mike's answer to my 2nd question). Can this claim also be extended for double quotes? Thanks, Zhao mike ledoux wrote: The backslash on line 3 is interpreted by the shell as a line continuation, so yes, it is just for increased readability by letting you break a long command line over multiple lines. It is not needed at the end of line 2 because the end of line 2 is inside of single quotes. To the shell, anything inside of single quotes is a single argument, so all of the spaces, newlines, etc. in there are passed without shell interpretation. ___ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/
Re: question: text substitution using Perl
Zhao Peng writes:
Just one more question:
To the shell, anything inside of single quotes is a single argument,
so all of the spaces, newlines, etc. in there are passed without shell
interpretation (quoted from the Mike's answer to my 2nd question).
Can this claim also be extended for double quotes?
Yes, but you need to understand that single quotes have different
semantics than double quotes.
These are all different:
$FOO
$FOO
'$FOO'
In double quotes variable interpolation is performed. You would do
well to understand all of these scenerios:
(look at how FOO is quoted at the end of each line)
$ FOO=zero one two three four
###
$ perl -le '$i=0; print \n\nNumber of command line arguments: ,
scalar(@ARGV); map { print argv[$i] = :, $ARGV[$i++], :; } @ARGV' FOO
Number of command line arguments: 1
argv[0] = :FOO:
$ perl -le '$i=0; print \n\nNumber of command line arguments: ,
scalar(@ARGV); map { print argv[$i] = :, $ARGV[$i++], :; } @ARGV' '$FOO'
Number of command line arguments: 1
argv[0] = :$FOO:
###
$ perl -le '$i=0; print \n\nNumber of command line arguments: ,
scalar(@ARGV); map { print argv[$i] = :, $ARGV[$i++], :; } @ARGV' $FOO
Number of command line arguments: 5
argv[0] = :zero:
argv[1] = :one:
argv[2] = :two:
argv[3] = :three:
argv[4] = :four:
###
$ perl -le '$i=0; print \n\nNumber of command line arguments: ,
scalar(@ARGV); map { print argv[$i] = :, $ARGV[$i++], :; } @ARGV' $FOO
Number of command line arguments: 1
argv[0] = :zero one two three four:
###
There's another kind of quote too: backquotes, `like these`. These
are different than either single or double quotes too.
Perhaps the best place you could learn all of the rules here is the
bash man page.
Regards,
--kevin
--
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alumni.unh.edu!kdc -- Tom Waits
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Re: question: text substitution using Perl
On Monday, Oct 23rd 2006 at 14:55 -0400, quoth Zhao Peng: =Mike and Kevin, = =Thank you both for quick, detailed, and crystal clarification and =explanation. = =Just one more question: = =To the shell, anything inside of single quotes is a single argument, so all =of the spaces, newlines, etc. in there are passed without shell =interpretation (quoted from the Mike's answer to my 2nd question). = =Can this claim also be extended for double quotes? = =Thanks, =Zhao = =mike ledoux wrote: = = The backslash on line 3 is interpreted by the shell as a line = continuation, so yes, it is just for increased readability by = letting you break a long command line over multiple lines. It is = not needed at the end of line 2 because the end of line 2 is inside = of single quotes. To the shell, anything inside of single quotes is = a single argument, so all of the spaces, newlines, etc. in there are = passed without shell interpretation. The difference between single quotes and double quotes is strictly whether variable interpolation occurs. foo=hello echo foo echo $foo echo '$foo' echo $foo is a 5 letter word Just for fun, this is a good example to help understand. echo $PATH ssh goofy echo $PATH ssh goofy 'echo $PATH' The first will tell you your path. The second will tel goofy to you your path and the third will tell goofy to tell you your path on goofy because what will be seen by goofy will be $PATH instead of the expansion of $PATH. -- A: Maybe because some people are too annoyed by top-posting. Q: Why do I not get an answer to my question(s)? A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? ___ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/
question: text substitution using Perl
Hi, Being a slow Perl learner who hasn't make much progress, I'm wondering if some kind souls could help me achieve the following 2 text substitution tasks detailed below. substitution 1 The characteristic of original string: 1, always start with $ 2, then followed by an integer, could be more than 1 digit, such as 23 3, always end with a period . 4, there is always a blank before after original string For example: $2. The characteristic of target string: always has ebcdic inserted into the original string between $ and the integer For example: $ebcdic2. So the substitution will look like this $2. - $ebcdic2. $67. - $ebcdic67. Should the regular expression for original string be: \$\d+\. ? substitution 2 The characteristic of original string: 1, always start with an integer, could be more than 1 digit, such as 23 2, then end with a period . 3, there is always a blank before after original string For example: 2. The characteristic of target string: always has s370ff added to the beginning of original string For example: s370ff2. So the substitution will look like this 2. - s370ff2. 14. - s370ff14. Should the regular expression for original string be: \d+\. ? My real situation is that I have a bunch of files at one directory, of which for the files whose name contained readme, I need to do 2 substitutions described above. Any input (and your time) is greatly appreciated. Thanks, Zhao ___ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/
Re: question: text substitution using Perl
Zhao Peng writes:
substitution 1
The characteristic of original string:
1, always start with $
2, then followed by an integer, could be more than 1 digit, such as 23
3, always end with a period .
4, there is always a blank before after original string
For example: $2.
The characteristic of target string: always has ebcdic inserted into
the original string between $ and the integer
For example: $ebcdic2.
So the substitution will look like this
$2. - $ebcdic2.
$67. - $ebcdic67.
Should the regular expression for original string be: \$\d+\. ?
Looks pretty much right to me.
I would make this replacement like this:
s/ \$(\d+)\. / \$ebcdic${1}. /g;
The ${1} in there is something called a backreference.
The 'g' at the end of the line generally specifies do this as many
times as possible on each line.
substitution 2
The characteristic of original string:
1, always start with an integer, could be more than 1 digit, such as 23
2, then end with a period .
3, there is always a blank before after original string
For example: 2.
The characteristic of target string:
always has s370ff added to the beginning of original string
For example: s370ff2.
So the substitution will look like this
2. - s370ff2.
14. - s370ff14.
Should the regular expression for original string be: \d+\. ?
I would make this replacement like this:
s/ (\d+)\. / s370ff${1}. /g;
My real situation is that I have a bunch of files at one directory, of
which for the files whose name contained readme, I need to do 2
substitutions described above.
One way to quickly do this might be like this:
perl -i.bak \
-pe 's/ \$(\d+)\. / \$ebcdic${1}. /g;
s/ (\d+)\. / s370ff${1}. /g;' \
your-directory-somewhere/*readme*
This in itself makes a backup for you, but you might want to make your
own backup files beforehand.
Just another Perl hacker,
--kevin
--
GnuPG ID: B280F24E Never could stand that dog.
alumni.unh.edu!kdc -- Tom Waits
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