Regular Expressions 101

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/
(\A|[^[])\[{2}[^[](?<content>[^]]+)[^]]\]{2}([^]]|\z)
/
gs
1st Capturing Group
(\A|[^[])
1st Alternative
\A
\A asserts position at start of the string
2nd Alternative
[^[]
Match a single character not present in the list below
[^[]
[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
\[
matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
{2} matches the previous token exactly 2 times
Match a single character not present in the list below
[^[]
[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
Named Capture Group content
(?<content>[^]]+)
Match a single character not present in the list below
[^]]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
Match a single character not present in the list below
[^]]
] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
\]
matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
{2} matches the previous token exactly 2 times
3rd Capturing Group
([^]]|\z)
1st Alternative
[^]]
Match a single character not present in the list below
[^]]
] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
2nd Alternative
\z
\z asserts position at the end of the string
Global pattern flags
g modifier: global. All matches (don't return after first match)
s modifier: single line. Dot matches newline characters
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gs

Test String