Grep Returns Nothing

Question

I feel like beating my head off my desk so any help is appreciated.

#!/bin/bash
echo 'wahegru.com mail is handled by 1 ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT4.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT1.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT3.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT2.ASPMX.L.GOOGLE.com.' |  grep '(?<=10 )(\.*+\w)*+'

Now I know this regex works:

(?<=10 )(\.*+\w)*+

Because I can paste the text into https://regex101.com/ and the regex and it finds what I need as a full match.

Why doesn't it work for grep though?

Edit, tried this:

echo 'wahegru.com mail is handled by 1 ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT4.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT1.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT3.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT2.ASPMX.L.GOOGLE.com.' |  grep -P '(?<=10 )(\.*+\w)*+'

But it gives me this:

usage: grep [-abcDEFGHhIiJLlmnOoqRSsUVvwxZ] [-A num] [-B num] [-C[num]]
[-e pattern] [-f file] [--binary-files=value] [--color=when]
[--context[=num]] [--directories=action] [--label] [--line-buffered]
[--null] [pattern] [file ...]

Tried

echo 'wahegru.com mail is handled by 1 ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT4.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT1.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT3.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT2.ASPMX.L.GOOGLE.com.' | perl -nle'print if m{(?<=10 )(\.*+\w)*+}'

But that outputs the wrong stuff:

wahegru.com mail is handled by 1 ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT4.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT1.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT3.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT2.ASPMX.L.GOOGLE.com.

To enable PCRE patterns, pass `-P` option, `grep -P 'pattern'`. To get the matched text, also use `o`: `grep -oP 'pattern'`, see [this demo](https://ideone.com/tB5bUH). — Wiktor Stribiżew, Jan 16 '19 at 11:45
For some reason, that doesn't work on my mac. See edit for my output. — Jack Robson, Jan 16 '19 at 11:49
[macos - grep -P no longer works how can I rewrite my searches - Stack Overflow](https://stackoverflow.com/questions/16658333/grep-p-no-longer-works-how-can-i-rewrite-my-searches) — user202729, Jan 16 '19 at 11:50
Thanks but that didn't work either. Please see additional edit. — Jack Robson, Jan 16 '19 at 11:53
Try `grep -o '10 [^ .]*$\.[^ .]*$*' | sed 's/^10 //;s/\.\.*$//'` — Wiktor Stribiżew, Jan 16 '19 at 11:53
Nice one Wiktor that works as expected. Do you how to output only the first match? — Jack Robson, Jan 16 '19 at 11:54
Add `| head -1`. If there cannot be consecutive dots, use `grep -o '10 [^ .]*$\.[^ .][^ .]*$*' | head -1 | sed 's/^10 //'`, see [this demo](https://ideone.com/NJmrn0). — Wiktor Stribiżew, Jan 16 '19 at 11:55
Legendary! I'd have been here all week figuring this out. Thank you so much everyone, especially you Wiktor! :) — Jack Robson, Jan 16 '19 at 11:56

Wiktor Stribiżew · Accepted Answer · 2019-01-16T11:59:57.053

1

You may use

echo 'wahegru.com mail is handled by 1 ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT4.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT1.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 10 ALT3.ASPMX.L.GOOGLE.com. wahegru.com mail is handled by 5 ALT2.ASPMX.L.GOOGLE.com.' | \ 
  grep -o '10 [^ .]*\(\.[^ .][^ .]*\)*' | head -1 | sed 's/^10 //'

See the grep demo online

Details

o option extracts the matches found in the input string
10 [^ .]*$\.[^ .][^ .]*$* matches
- 10 - a literal substring
- [^ .]* - 0+ chars other than space and .
- $\.[^ .][^ .]*$* - 0 or more repetitions of
  - \. - a dot
  - [^ .][^ .]* - 1 or more chars other than . and space
head -1 - gets the first match
sed 's/^10 //' removes the initial 10 substring.

edited Jan 16 '19 at 11:59

answered Jan 16 '19 at 11:57

Wiktor Stribiżew

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You seem to be an expert. Do you know why it gives the wrong answer here? – Jack Robson Jan 16 '19 at 11:59
echo 'zzzzzz.org.uk mail is handled by 10 d208222.a.ess.uk.barracudanetworks.com. eothenhomes.org.uk mail is handled by 20 d208222.b.ess.uk.barracudanetworks.com.' | sed 's/^10 //;s/\.\.*$//' | head -1 – Jack Robson Jan 16 '19 at 12:00
Impressive. Thanks. – Jack Robson Jan 16 '19 at 12:02

Grep Returns Nothing

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