"mmmnnn" -match "(?<orz>mmm)(?(orz)nnn|1+)"; $Matches

 

"aab" -match "^(?=aa)aaa"
#False
"aab" -match "^(?=aa)aab"
#True

 

"abc" -match "^(?=ab)abc$"
#True
"abc" -match "^(?=abc)abc$"
#True
"abc" -match "(?=abc)abc$"
#True

"abc" -match "a(?=bc)bc$"

#True

 

"123ABC" -MATCH "(?((?<ABC>DEF))123|123)(?(ABC)(?<SEC>ABC))"; $Matches

#The group ABC failed matching first and then group SEC wouldn't be executed for matching. The second ABC is not an expression.
#True

#Name                           Value
#    ----                              -----
#       0                              123

"123ABC" -MATCH "(?((?<ABCD>DEF))123|123)(?(ABC)(?<SEC>ABC))"; $Matches

#ABCD failed and ABC is executed for matching as an expression.
#True

#Name                           Value
#    ----                              -----
#  SEC                            ABC
#       0                       123ABC