Regex lookahead lookbehind and atomic groups

Question

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    lt    - positive lookbehind    lt    - negative lookbehind     gt   - atomic group

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Lookarounds are zero width assertions  They check for a regex  towards right or left of the current position - based on ahead or behind   succeeds or fails when a match is found  based on if it is positive or negative  and discards the matched portion  They don t consume any character - the matching for regex following them  if any   will start at the same cursor position    Read regular-expression info for more details    Positive lookahead      Syntax      REGEX 1 REGEX 2   Match only if REGEX 1 matches  after matching REGEX 1  the match is discarded and searching for REGEX 2 starts at the same position   example       a-z0-9  4    a-z  1 2  0-9  2 3    REGEX 1 is  a-z0-9  4   which matches four alphanumeric chars followed by end of line  REGEX 2 is  a-z  1 2  0-9  2 3  which matches one or two letters followed by two or three digits   REGEX 1 makes sure that the length of string is indeed 4  but doesn t consume any characters so that search for REGEX 2 starts at the same location  Now REGEX 2 makes sure that the string matches some other rules  Without look-ahead it would match strings of length three or five    Negative lookahead   Syntax      REGEX 1 REGEX 2   Match only if REGEX 1 does not match  after checking REGEX 1  the search for REGEX 2 starts at the same position   example         bFWORD b  w 10 30     The look-ahead part checks for the FWORD in the string and fails if it finds it  If it doesn t find FWORD  the look-ahead succeeds and the following part verifies that the string s length is between 10 and 30 and that it contains only word characters a-zA-Z0-9   Look-behind is similar to look-ahead  it just looks behind the current cursor position  Some regex flavors like javascript doesn t support look-behind assertions  And most flavors that support it  PHP  Python etc  require that look-behind portion to have a fixed length    Atomic groups basically discards forgets the subsequent tokens in the group once a token matches  Check this page for examples of atomic groups

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Grokking lookaround rapidly  How to distinguish lookahead and lookbehind   Take 2 minutes tour with me        - positive lookahead    lt    - positive lookbehind   Suppose       A  B  C  in a line   Now  we ask B  Where are you  B has two solutions to declare it location    One  B has A ahead and has C bebind Two  B is ahead lookahead  of C and behind  lookhehind  A     As we can see  the behind and ahead are opposite in the two solutions  Regex is solution Two

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Examples  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     lt  foo bar    finds the 1st bar   bar  which has  foo  before it     lt  foo bar    finds the 2nd bar   bar  which does not have  foo  before it    You can also combine them      lt  foo bar   bar     finds the 1st bar   bar  with  foo  before it and  bar  after it    Definitions  Look ahead positive       Find expression A where expression B follows   A   B    Look ahead negative       Find expression A where expression B does not follow   A   B    Look behind positive    lt     Find expression A where expression B precedes      lt  B A   Look behind negative    lt     Find expression A where expression B does not precede      lt  B A   Atomic groups    gt    An atomic group exits a group and throws away alternative patterns after the first matched pattern inside the group  backtracking is disabled        gt 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 disabled   A 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    match its 1st alternative foo  then fail as s does not immediately follow in foots  and backtrack to its 2nd alternative  match its 2nd alternative foot  then succeed as s immediately follows in foots  and stop     Some resources   http   www regular-expressions info lookaround html http   www rexegg com regex-lookarounds html   Online testers   https   regex101 com

[regex] Regex lookahead, lookbehind and atomic groups

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