Based on the output to get value with regular expression-CodePudding

There's an output like this:

RA#show segment-routing traffic-eng on-demand color detail | utility egrep Color -B 10
Sat Dec 25 11:24:22.891 JST

SR-TE On-Demand-Color database
------------------------

On-Demand Color: 20
--
 Performance-measurement:
   Reverse-path Label: Not Configured
   Delay-measurement: Disabled
   Liveness-detection: Enabled 《-------
     Profile: liveness1
     Invalidation Action: down
     Logging:
       Session State Change: Yes
 Per-flow Information:
  Default Forward Class: 0
On-Demand Color: 23
--
 Performance-measurement:
   Reverse-path Label: Not Configured
   Delay-measurement: Disabled
   Liveness-detection: Enabled  《--------
     Profile: liveness1
     Invalidation Action: down
     Logging:
       Session State Change: Yes
 Per-flow Information:
  Default Forward Class: 0
On-Demand Color: 301

regex "On-Demand Color:\s(\S )" can be used to extract color "20,23,301", but as Liveness-detection was not enabled under "On-Demand Color: 301", So I expected only color "20" and "23" can be extacted. Is it possible to achieve this by regular expression?

CodePudding user response：

Yes, it is possible to do this with regex, although beware that a parser would probably be better in this scenario. The idea is to search for the string that you want as long as it has a "Liveness-detection: Enabled" afterwards.

The regex would look like this:

On-Demand Color:\s(\S )\n.*\n.*\n.*\n.*\n.*Liveness-detection: Enabled

Demo: https://regex101.com/r/SElvt2/1

And basically it matches also a few lines after you find the "On-Demand Color:" string. This regex can further be simplified to this:

On-Demand Color:\s(\S )(\n.*?)*Liveness-detection: Enabled

Demo: https://regex101.com/r/857JQM/1

So that the "Liveness-detection:" string can be a few lines before of after the expected position.

CodePudding user response：

For your 2nd method, this might be better.

On-Demand Color:\s(\S )(?:\n.*){5}Liveness-detection: Enabled

CodePudding user response：

If you want to be a bit more flexible and if the value can be present after n number of lines instead of exactly 5, you can make use of a negative lookahead.

This will also prevent overmatching if Liveness-detection: Enabled is not present in the current On-Demand Color: part.

On-Demand Color:\s(\S )(?:\n(?!On-Demand Color:|\s*Liveness-detection:).*)*\n\s*Liveness-detection: Enabled

The pattern matches:

On-Demand Color: Match the starting string
\s(\S ) Match a whitespace char and capture 1 non whitespace chars (or use (\d )for digits) in capture group 1
(?: Non capture group
- \n(?!On-Demand Color:|\s*Liveness-detection:)
- .* Match the whole line
)* Close non capture group and optionally repeat
\n\s*Liveness-detection: Enabled Match a newline, optional whitespace chars and the string that you want to match

Regex demo