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I found these things in my regex body but I haven't got a clue what I can use them for. Does somebody have examples so I can try to understand how they work?
(?!) - negative lookahead (?=) - positive lookahead (?<=) - positive lookbehind (?<!) - negative lookbehind (?>) - atomic group 
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Lookahead allows to add a condition for “what follows”. Lookbehind is similar, but it looks behind. That is, it allows to match a pattern only if there's something before it.
The good news is that you can use lookbehind anywhere in the regex, not only at the start.
An atomic group is a group that, when the regex engine exits from it, automatically throws away all backtracking positions remembered by any tokens inside the group. Atomic groups are non-capturing.
Unlike look-ahead, look-behind is used when the pattern appears before a desired match. You're “looking behind” to see if a certain string of text has the desired pattern behind it. If it does, then that string of text is a match.
Given the string foobarbarfoo:
bar(?=bar) finds the 1st bar ("bar" which has "bar" after it) bar(?!bar) finds the 2nd bar ("bar" which does not have "bar" after it) (?<=foo)bar finds the 1st bar ("bar" which has "foo" before it) (?<!foo)bar finds the 2nd bar ("bar" which does not have "foo" before it) You can also combine them:
(?<=foo)bar(?=bar) finds the 1st bar ("bar" with "foo" before it and "bar" after it) (?=) Find expression A where expression B follows:
A(?=B) (?!) Find expression A where expression B does not follow:
A(?!B) (?<=) Find expression A where expression B precedes:
(?<=B)A (?<!) Find expression A where expression B does not precede:
(?<!B)A (?>) An atomic group exits a group and throws away alternative patterns after the first matched pattern inside the group (backtracking is disabled).
(?>foo|foot)s applied to foots will match its 1st alternative foo, then fail as s does not immediately follow, and stop as backtracking is disabledA non-atomic group will allow backtracking; if subsequent matching ahead fails, it will backtrack and use alternative patterns until a match for the entire expression is found or all possibilities are exhausted.
(foo|foot)s applied to foots will:
foo, then fail as s does not immediately follow in foots, and backtrack to its 2nd alternative;foot, then succeed as s immediately follows in foots, and stop.
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