Split Lines in a variable and compare function

I have a variable called $a which has multiple lines as follows:

Source

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saturday:activity1

saturday:activity2

saturday:activity3

i have another variables called $b which also has multiple lines as follows

Files

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[string1] text and strings

[string2] text and strings

[string3] text and strings

[string4] text and strings

I need a function that only takes what is in the [ ] from each line in $b, something like this:

Split(']')[0].TrimStart('[') # this needs to apply to each line in $b

and compares that to every activity in $ (which comes after the colon in each line in $a)

so all lines in $a need to be split, something like this: $_.Split(':') #applies to each line in $a

if there is at least one match across all lines, i want the outcome to say "yes", if there was no match at all, the outcome to be "No",

Please note that the word Saturday does exist in each line in $a.