For a programming exercise, I am mean to design a Perl script that detects double letters in a text file.
I tried the following expressions
Im aware that the + means to search for one or more occurences of that character, however trying both of these did not meet the requirements of my program.
Also
did not prove to be helpful as well
After doing some searching, I stumbled across the correct form of the regex for the double letter case. It turned out to be
Now I know that . refers to any single character and the \1 refers to the first character in the line being read (if s/..../.... is being used), but Im still puzzled as to why /(.)\1/ works instead of /[a-zA-Z]+/ for the case of double letters ?
Now I know that . refers to any single character and the \1 refers to the first character in the line being read (if s/..../.... is being used), but Im still puzzled as to why /(.)\1/ works instead of /[a-zA-Z]+/ for the case of double letters ?
* Incorrect text removed *
/[a-zA-Z]+/ only means matching a contiguous sequence of letters, so not only 'AA' or 'zz' will match, 'Az' will match too.
Last edited by cbkihong; 07-24-2008 at 02:28 AM..
Reason: Incorrect text removed
\1 is a backreference to what is matched in the parenthesis in the regexp. So /(.)\1/ finds a double occurance of whatever (.) matched. It is similar to $1 but is used inside the regexp. It is discussed in some detail here:
Actually, not even /(.)\1/ is correct. In Perl, you should use /(.)$1/. The former syntax is there for compatibility with I think awk or sed but that should in general not be used in Perl, because Perl has more uses of backslash that may interfere with backtracking.
That is not correct. Using \1 is perfectly good perl code. \1 and $1 really have two seperate uses. See the link I posted in my previous post. A short test shows they do not do the same thing:
because it means one or more of the characters inside the square brackets, any of the characters, in any order. You want to find two of the same character repeated in a string, not one or more of any character inside the [] brackets.
Interesting and thoughtful question. You use "(" and ")" to mark (remember) a pattern and recall the remembered pattern with "\" followed by a single digit (back reference).
In your particular case, "(.)\1" means remember a character and recall the character.
You can extend this method to find words with multiple double letters. '(.)\1(.)\2(.)\3' will match any word with three double letters, e.g. bookkeeper.
Experts -
I found a script on one of the servers that I work on and I need help understanding
one of the lines.
I know what the script does, but I'm having a hard time understanding the grouping.
Can someone help me with this?
Here's the script...
#!/usr/bin/perl
use strict;
use... (2 Replies)
I am not a big expert in regex and have just little understanding of that language.
Could you help me to understand the regular Perl expression:
^(?!if\b|else\b|while\b|)(?:+?\s+){1,6}(+\s*)\(*\) *?(?:^*;?+){0,10}\{
------
This is regex to select functions from a C/C++ source and defined in... (2 Replies)
Could anyone please make me understand how the ?= works below ..
After executing this I am getting the same output.
$string="I love chocolate.";
$string =~ s/chocolate(?= ice)/vanilla/;
print "$string\n"; (2 Replies)
Hello,
I'm trying to get a quick help on regex since i'm not a regular programmer.
Below is the line i'm trying to apply my regex to..i want to use the regex in a for loop and this line will keep on changing.
subject=... (4 Replies)
Hi Guys
I have the following regex
$OSRELEASE = $1 if ($output =~ /(Mac OS X (Server )?10.\d)/);
output is currently
Mac OS X 10.7.5
when the introduction of Mac 10.8 output changes to
OS X 10.8.2
they have dropped the Mac bit so i changed the regex to be (2 Replies)
HI,
I'm new to perl and need simple regex for reading a file using my perl script.
The text file reads as -
filename=/pot/uio/current/myremificates.txt
certificates=/pot/uio/current/userdir/conf/user/gamma/settings/security/... (3 Replies)
Hi,
I get the following when I cat a file *.log
xxxxx
=====
dasdas gwdgsg fdsagfsag agsdfag
=====
random data
=====
My output should look like :
If the random data after the 2nd ==== is null then OK should be printed else
the random data should be printed.
How do I go about this... (5 Replies)
I am having trouble parsing rpm filenames in a shell script.. I found a snippet of perl code that will perform the task but I really don't have time to rewrite the entire script in perl. I cannot for the life of me convert this code into something sed-friendly:
if ($rpm =~ /(*)-(*)-(*)\.(.*)/)... (1 Reply)
I have got numbers like
l255677
l376039
l188144
l340482
l440700
l254113
to match the numbers starting with '13' what would be the regex
=~/13(.*)/ =======>This is not working ....
But for user123,user657
regex =~/user(.*)/ ========>works
Thanks for help..!! (7 Replies)
Hi,
Can anyone help me to find regular expression for the following in Perl?
"The string can only contain lower case letters (a-z) and no more than one of any letter."
For example: "table" is accepted, whether "dude" is not.
I have coded like this:
$str = "table";
if ($str =~ m/\b()\b/) {... (4 Replies)