Friday, October 7, 2022

[SOLVED] Simple grep regex to catch [0 0] but not [00]

Issue

Buongiorno,

I am trying to grep all the patterns that look like: [00 0] or [ 0000] but not [00] or [0000] : there has to be at least one space.

Naively, I thought that

grep -P '\[.* .*\]'

would do it ; when I copy paste the regex in regex101 with some test cases, I get the following:

unmatched:
0 0
000
[00]
[0]
[00]

matched:
[0 0]
[ 0]
[0 ]
[  0  ]
[ 0 ]

which is exactly what I want.

But grep seems to match also the things between brackets but with no space:

[00]
[0]
[00]

What am I missing ?

\[0* 0*\]

seems to work but I'm going to have cases where it's not only 0...


Solution

This grep would work with BRE (basic regex), it doesn't require ERE (extended regex) or experimental -P (perl regex):

grep '\[[^] ]* [^]]*]' file

[0 0]
[ 0]
[0 ]
[  0  ]
[ 0 ]

If you have only zeroes and spaces inside the [...] then try:

grep '\[0* [0 ]*]' file

Online Demo

RegEx Details:

  • \[: Match a [
  • [^] ]*: Match 0 or more of any chars that are not ] and space
  • : Match a space
  • [^]]*: Match 0 or more of any chars that are not ]
  • ]: Match a ]

Problem with .* is that it matches 0 or more of any character except a line break. It won't stop at ].



Answered By - anubhava
Answer Checked By - Mary Flores (WPSolving Volunteer)
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