Regular Expressions 101

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/
(?sm)interface (\S*)(?:(?!^!$).)*port-security.*?!$
/
gm
(?sm)
match the remainder of the pattern with the following effective flags: gms
s modifier: single line. Dot matches newline characters
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
interface
matches the characters interface literally (case sensitive)
1st Capturing Group
(\S*)
\S
matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:(?!^!$).)*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?!^!$)
Assert that the Regex below does not match
^ asserts position at start of a line
! matches the character ! with index 3310 (2116 or 418) literally (case sensitive)
$ asserts position at the end of a line
. matches any character
port-security
matches the characters port-security literally (case sensitive)
.
matches any character
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
! matches the character ! with index 3310 (2116 or 418) literally (case sensitive)
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String