Singleline RegEx issue

Question

I'm struggling with a RegEx expression. I like to match a text similar to the example below.

The text consist of blocks, starting always with wlan. The RegEx should only match, if the line dot1 authent exists in the block.

The SSIDx behind wlan should match with the 1st RegEx group. The word behind dot1 authent should be match in the 2nd RegEx group.

Group 1	Group 2	Result
SSID1	TESTTEST1	OK
SSID3	TESTTEST3	OK
SSID4	TESTTEST4	OK

wlan SSID1 SSID1
 test2
 test1
 dot1 authent TESTTEST1
 test3
wlan SSID2 SSID2
 test21
 test11
 test31
wlan SSID3 SSID3
 test22
 test12
 dot1 authent TESTTEST3
 test32
wlan SSID4 SSID4
 test23
 test13
 dot1 authent TESTTEST4
 test33

The following RegEx expression does match almost the desired content.

(?s:wlan (.+?)\s.+?dot1 authent (.+?)\n)

Unfortunately in case a wlan group doesn't contain the line dot1 authent, the RegEx match the following wlan group causing a wrong match.

In the example the matches are the following:

Group1	Group 2	Result
SSID1	TESTTEST1	OK
SSID2	TESTTEST3	NOT OK
SSID4	TESTTEST4	OK

SSID2 should not match as dot1 authent is not defined for this group. Instead, it should match SSID3.

I added (?!wlan) in the RegEx, but this didn't have any effect.

(?s:wlan (.+?)\s.+?(?!wlan)dot1 authent (.+?)\n)

Can anyone give me a hint, what I did wrong and how to achieve this match?

Many thanks

Answer 1

You can use a negative lookahead to prevent crossing the lines that make it a separate block and match the dot1 authend with spaces to the left and right at least once per block.

^wlan (\S+).*(?:\r?\n(?!wlan|.*? dot1 authent ).*)*\r?\n.*? dot1 authent (.+).*(?:\r?\n(?!wlan).*)*

See a regex demo

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Or only with \n and word boundaries \b for the dot1 authent match, and stop matching after the first occurrence of the group 2 value:

^wlan (\S+).*(?:\n(?!wlan|.*?\bdot1 authent\b).*)*\n.*?\bdot1 authent\b(.+)

Regex demo

Answer 2

I'm assuming you're not processing the entire file at once; you've at least split the strings such that each wlan is the beginning of each string. Otherwise, we're going to have to come up with a new regex.

Try this regex:

(?s:wlan (.+?)(?=\n +dot1 authent (.+?)))

Expressed as a Perl regex:

/^wlan (.+?)(?=\n +dot1 authent (.+?))/s

This uses a lookaround to figure out if there's an indented dot1 authent line ahead in the string.