Replace preg replace e modifier with preg replace callback

Question

I m terrible with regular expressions  I m trying to replace this   public static function camelize  word       return preg replace          a-z   e    strtoupper    2      word       with preg replace callback with an anonymous function  I don t understand what the   2 is doing  Or for that matter exactly how preg replace callback works    What would be the correct code for achieving this

User · Answer

You shouldn t use flag e  or eval in general    You can also use T-Regx library  pattern         a-z    - gt replace  word - gt by  - gt group 2 - gt callback  strtoupper

User · Answer

In a regular expression  you can  capture  parts of the matched string with  brackets   in this case  you are capturing the       and   a-z   parts of the match  These are numbered starting at 1  so you have back-references 1 and 2  Match 0 is the whole matched string   The  e modifier takes a replacement string  and substitutes backslash followed by a number  e g   1  with the appropriate back-reference - but because you re inside a string  you need to escape the backslash  so you get    1   It then  effectively  runs eval to run the resulting string as though it was PHP code  which is why it s being deprecated  because it s easy to use eval in an insecure way    The preg replace callback function instead takes a callback function and passes it an array containing the matched back-references  So where you would have written    1   you instead access element 1 of that parameter - e g  if you have an anonymous function of the form function  matches           the first back-reference is  matches 1  inside that function   So a  e argument of    do stuff   1     and    do stuff   2     could become a callback of   function  m    return do stuff  m 1      and    do stuff  m 2        Or in your case   strtoupper    2      could become  function  m    return strtoupper  m 2        Note that  m and  matches are not magic names  they re just the parameter name I gave when declaring my callback functions  Also  you don t have to pass an anonymous function  it could be a function name as a string  or something of the form array  object   method   as with any callback in PHP  e g   function stuffy callback  things        return do stuff  things 1      and    do stuff  things 2       foo   preg replace callback     a-z    and   a-z        stuffy callback    fish and chips      As with any function  you can t access variables outside your callback  from the surrounding scope  by default  When using an anonymous function  you can use the use keyword to import the variables you need to access  as discussed in the PHP manual  e g  if the old argument was    do stuff   1   foo     then the new callback might look like  function  m  use   foo    return do stuff  m 1    foo       Gotchas   Use of preg replace callback is instead of the  e modifier on the regex  so you need to remove that flag from your  pattern  argument  So a pattern like  blah    blah mei would become  blah    blah mi  The  e modifier used a variant of addslashes   internally on the arguments  so some replacements used stripslashes   to remove it  in most cases  you probably want to remove the call to stripslashes from your new callback

User · Answer

preg replace shim with eval support  This is very inadvisable  But if you re not a programmer  or really prefer terrible code  you could use a substitute preg replace function to keep your  e flag working temporarily          Can be used as a stopgap shim for preg replace   calls with  e flag     Is likely to fail for more complex string munging expressions  And    very obviously won t help with local-scope variable expressions         license  CC-BY-   -comment-must-be-retained     security  Provides  eval  support for replacement patterns  Which      poses troubles for user-supplied input when paired with overly      generic placeholders  This variant is only slightly stricter than      the C implementation  but still susceptible to varexpression  quote      breakouts and mundane exploits from unquoted capture placeholders      url  https   stackoverflow com q 15454220     function preg replace eval  pattern   replacement   subject   limit -1          strip  e flag      pattern   preg replace     W a-df-z   e  a-df-z     i     1 2    pattern         warn about most blatant misuses at least     if  preg match                pattern             trigger error  preg replace eval    regex contains      or      placeholders  which easily causes security issues for unconstrained user input in the replacement expression  Transform your code to use preg replace callback   with a sane replacement callback                 run preg replace with eval-callback     return preg replace callback           pattern          function   matches  use   replacement                  substitute  1  2     with literals from  matches                repl   preg replace callback                       lt                     d                      function   m  use   matches                        if   isset  matches  m 1       trigger error  No capture group for   m 0   eval placeholder                          return addcslashes  matches  m 1                0      additionally escapes     and backticks                                     replacement                              run the replacement expression             return eval  return  repl                         subject           limit            In essence  you just include that function in your codebase  and edit  preg replace to preg replace eval wherever the  e flag was used   Pros and cons    Really just tested with a few samples from Stack Overflow  Does only support the easy cases  function calls  not variable lookups   Contains a few more restrictions and advisory notices  Will yield dislocated and less comprehensible errors for expression failures  However is still a usable temporary solution and doesn t complicate a proper transition to preg replace callback  And the license comment is just meant to deter people from overusing or spreading this too far    Replacement code generator  Now this is somewhat redundant  But might help those users who are still overwhelmed with manually restructuring their code to preg replace callback  While this is effectively more time consuming  a code generator has less trouble to expand the  e replacement string into an expression  It s a very unremarkable conversion  but likely suffices for the most prevalent examples   To use this function  edit any broken preg replace call into preg replace eval replacement and run it once  This will print out the according preg replace callback block to be used in its place          Use once to generate a crude preg replace callback   substitution  Might often    require additional changes in the  return       expression  You ll also have to    refit the variable names for input output obviously         gt  gt  gt   preg replace eval replacement    w      strtopupper   1      ignored       function preg replace eval replacement  pattern   replacement   subjectvar  IGNORED          pattern   preg replace     W a-df-z   e  a-df-z     i     1 2    pattern        replacement   preg replace callback             lt                     d             function   m    return    m   m 1          replacement        ve    var export        bt   debug backtrace 0  1  0       print   lt pre gt  lt code gt       ----------------------------------------------------       replace preg     call in   bt file   line  bt line  with       ----------------------------------------------------       OUTPUT VAR   preg replace callback            ve  pattern  TRUE            function    m                return   replacement                        YOUR INPUT VARIABLE GOES HERE            ----------------------------------------------------      lt  code gt  lt  pre gt  n       Take in mind that mere copy amp pasting is not programming  You ll have to adapt the generated code back to your actual input output variable names  or usage context    Specificially the  OUTPUT   assignment would have to go if the previous preg replace call was used in an if  It s best to keep temporary variables or the multiline code block structure though    And the replacement expression may demand more readability improvements or rework    For instance stripslashes   often becomes redundant in literal expressions  Variable-scope lookups require a use or global reference for within the callback  Unevenly quote-enclosed  - 1- 2  capture references will end up syntactically broken by the plain transformation into  - m 1 - m 2     The code output is merely a starting point  And yes  this would have been more useful as an online tool  This code rewriting approach  edit  run  edit  edit  is somewhat impractical  Yet could be more approachable to those who are accustomed to task-centric coding  more steps  more uncoveries   So this alternative might curb a few more duplicate questions

[php] Replace preg_replace() e modifier with preg_replace_callback

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