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/foo(?!.*foo)/

s/(?<=foo)/,/g; # Without lookbehind: s/foo/foo,/g or s/(foo)/$1,/g

s/(?<=look)(?=ahead)/-/g;

/foo  # Match starting at foo
 (         # Capture
...
 )         # End capture
 bar  # and ending at bar
/x;

s/(?<=,        # after a comma,
    (?!        # but not matching
...
      (?=\d)   #   digit afterward
    )
  )/ /gx;      # substitute a space

s/(?<=,        # after a comma, but either
    (?:
...
      (?!\d)   #   not matching digit afterward
    )
  )/ /gx;      # substitute a space

print "$1\n" while /(?=(.*))/g;