包含奇数管道字符的行的正则表达式

Question

I'm cleaning up a LaTeX file, and I'm in a situation where I need to distinguish absolute value |x| from the set "such that" symbol ie {x | x < 0}.

The first step for me is to find all lines containing an odd number of | characters (ie the pipe symbol).

In principle, I know how to do this, but I've tried the following regex command with no luck.

egrep '^[^\|]*\|([^\|]*\|[^\|]*\|)*[^\|]*$'

The idea is that a matching line contains, in order:

• The line start
• 0 or more non-pipe characters
• Exactly one pipe character
• 0 or more copies of text containing exactly 2 pipes
• The line end

However, for some reason this isn't working. I run the command on the following file:

\[
S = \{ x | x < 0}
y = |x|
\]

and none of the lines match.

I suspect I'm making a silly mistake somewhere, possibly to do with escaping the pipe characters, but I'm stumped as to what's wrong.

Can anybody tell me either how to fix this, or provide an alternate expression which matches lines containing an odd number of pipe characters?

Answer 1

Inside the [] , | is not a special character so should not be escaped by \\ . Try:

egrep '^[^|]*\|([^|]*\|[^|]*\|)*[^|]*$'

Answer 2

Better to use awk for this purpose:

awk -F '|' '!(NF%2)'

TESTING:

echo "a|bc|d|erg" | awk -F '|' '!(NF%2)'

OUTPUT:

a|bc|d|erg

echo "abc|d|ergxy" | awk -F '|' '!(NF%2)'

OUTPUT:

Answer 3

how about:

awk -F'|' 'NF&&(NF-1)%2' file

example:

kent$  cat file
|foo|bar
| | | | |
||||||
|||||||

kent$  awk -F'|' 'NF&&(NF-1)%2' file
| | | | |
|||||||

Answer 4

Perl, which is cross platform (Windows too) and generally installed everywhere these days, is my axe of choice:

perl -ne 'print if (s/\\|/\\|/g) %2 == 1' file

Answer 5

script.sed

#!/bin/sed -nf

# Save to hold
h

# Delete all non | chars
s@[^|]@@g

# Odd match
/^\(||\)*|$/ {
  # Fetch hold
  g
  s@^@odd\t:@
}

# Even match
/^\(||\)\+$/ {
  # Fetch hold
  g
  s@^@even\t:@
}

# No match
/^$/ {
  # Fetch hold
  g
  s@^@none\t:@
}

# Print
p

data.txt

do|odd
do|odd|match|me
|even match|me
do|even match|me
do|even match|also|me|please

no-match

shell

sed -nf script.sed data.txt

stdout

odd :do|odd
odd :do|odd|match|me
even    :|even match|me
even    :do|even match|me
even    :do|even match|also|me|please
none    :
none    :no-match