How to write regexp literal in match expression?












2















This question is about the proper way to write regular expression literals in a match expression under bash.



In zsh, the match below succeeds, as I expected:



% [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
0


Not so in bash:



$ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
1


I know that this match succeeds in bash



$ ( regexp='^ [0-9]+ $'; [[ ' 123 ' =~ $regexp ]]; echo $? )
0


...but it requires assigning the regular expression to an intermediate variable.





My question is: How does one write an arbitrary regular expression literal in a match expression under bash?










share|improve this question



























    2















    This question is about the proper way to write regular expression literals in a match expression under bash.



    In zsh, the match below succeeds, as I expected:



    % [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
    0


    Not so in bash:



    $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
    1


    I know that this match succeeds in bash



    $ ( regexp='^ [0-9]+ $'; [[ ' 123 ' =~ $regexp ]]; echo $? )
    0


    ...but it requires assigning the regular expression to an intermediate variable.





    My question is: How does one write an arbitrary regular expression literal in a match expression under bash?










    share|improve this question

























      2












      2








      2


      1






      This question is about the proper way to write regular expression literals in a match expression under bash.



      In zsh, the match below succeeds, as I expected:



      % [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
      0


      Not so in bash:



      $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
      1


      I know that this match succeeds in bash



      $ ( regexp='^ [0-9]+ $'; [[ ' 123 ' =~ $regexp ]]; echo $? )
      0


      ...but it requires assigning the regular expression to an intermediate variable.





      My question is: How does one write an arbitrary regular expression literal in a match expression under bash?










      share|improve this question














      This question is about the proper way to write regular expression literals in a match expression under bash.



      In zsh, the match below succeeds, as I expected:



      % [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
      0


      Not so in bash:



      $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
      1


      I know that this match succeeds in bash



      $ ( regexp='^ [0-9]+ $'; [[ ' 123 ' =~ $regexp ]]; echo $? )
      0


      ...but it requires assigning the regular expression to an intermediate variable.





      My question is: How does one write an arbitrary regular expression literal in a match expression under bash?







      bash shell-script scripting regular-expression syntax






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked Nov 21 '18 at 20:09









      kjokjo

      4,083103764




      4,083103764






















          2 Answers
          2






          active

          oldest

          votes


















          6














          It's best to put it in a variable, as recommended by the Bash reference manual 3.2.4.2 Conditional Constructs:




          Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. It is sometimes difficult to specify a regular expression literally without using quotes, or to keep track of the quoting used by regular expressions while paying attention to the shell’s quote removal. Using a shell variable to store the pattern decreases these problems.




          However in order to write it directly inside of the bash extended test you need to remove the quotes and escape the spaces:



          $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
          0





          share|improve this answer































            2














            In bash, the canonical answer is: use a var



            $ reg='^ [0-9]+ $'
            $ [[ ' 123 ' =~ $reg ]]; echo $?


            That works for spaces, backslash, and many other things:



            $ reg='^ 123 $'
            $ [[ ' 123 ' =~ $reg ]]; echo $?
            0


            If you want to write it as a literal you need to play (carefully) with quoting.

            You need to quote the parts that need to be literal and leave the parts that should be interpreted as a regex un-quoted. The space needs quoting. Here using backslah:



            $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
            0


            And here using double quotes:



            $ [[ ' 123 ' =~ ^" "[0-9]+" "$ ]]; echo $?


            But that "quoting" gets real messy with things like a backslash:



            $ [[ ' 123 ' =~ ^" "[0-9]+"\"[0-9]+" "$ ]]; echo $?
            0


            Simpler, safer:



            $ reg='^ [0-9]+\[0-9]+ $'
            $ [[ ' 123 ' =~ $reg ]]; echo $?
            0


            And, no, you don't need a subshell to do that (the parenthesis in your question).



            $ reg='^ [0-9]+\[0-9]+ $'; [[ ' 123 ' =~ $reg ]]; echo $?




            Yes, that may seem annoying.
            And, yes, the command you presented happens to work in zsh:



            $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
            0


            But quoting is always a problem (in any shell), what should happen to backslash ?:



            % [[ ' 123 ' =~ ' 123 ' ]]; echo $?
            zsh: failed to compile regex: Invalid back reference
            1


            % [[ ' 123 ' =~ '^ [0-9]+\3 $' ]]; echo $?
            0


            Double it !. It is not exactly: literal.






            share|improve this answer


























            • I know that the subshell is not needed for the match; I use subshells to avoid polluting my current shell's namespace with a variable that I need to use only once.

              – kjo
              Nov 21 '18 at 21:08













            • An unset regexp is always faster than a subshell. @kjo

              – Isaac
              Nov 21 '18 at 23:12











            • Maybe, but I don't care about the few milliseconds here and there. The subshell takes fewer keystrokes, it is robust to any number of changes I may do in the subshell (not just the setting of one variable), and there's no chance I will forget to run unset afterwards.

              – kjo
              Nov 22 '18 at 16:54













            • Sure, why fight the waste if it is easier?. Why reduce the use of plastics until we get a plastic planet? @kjo

              – Isaac
              Nov 23 '18 at 0:49











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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            6














            It's best to put it in a variable, as recommended by the Bash reference manual 3.2.4.2 Conditional Constructs:




            Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. It is sometimes difficult to specify a regular expression literally without using quotes, or to keep track of the quoting used by regular expressions while paying attention to the shell’s quote removal. Using a shell variable to store the pattern decreases these problems.




            However in order to write it directly inside of the bash extended test you need to remove the quotes and escape the spaces:



            $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
            0





            share|improve this answer




























              6














              It's best to put it in a variable, as recommended by the Bash reference manual 3.2.4.2 Conditional Constructs:




              Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. It is sometimes difficult to specify a regular expression literally without using quotes, or to keep track of the quoting used by regular expressions while paying attention to the shell’s quote removal. Using a shell variable to store the pattern decreases these problems.




              However in order to write it directly inside of the bash extended test you need to remove the quotes and escape the spaces:



              $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
              0





              share|improve this answer


























                6












                6








                6







                It's best to put it in a variable, as recommended by the Bash reference manual 3.2.4.2 Conditional Constructs:




                Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. It is sometimes difficult to specify a regular expression literally without using quotes, or to keep track of the quoting used by regular expressions while paying attention to the shell’s quote removal. Using a shell variable to store the pattern decreases these problems.




                However in order to write it directly inside of the bash extended test you need to remove the quotes and escape the spaces:



                $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
                0





                share|improve this answer













                It's best to put it in a variable, as recommended by the Bash reference manual 3.2.4.2 Conditional Constructs:




                Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. It is sometimes difficult to specify a regular expression literally without using quotes, or to keep track of the quoting used by regular expressions while paying attention to the shell’s quote removal. Using a shell variable to store the pattern decreases these problems.




                However in order to write it directly inside of the bash extended test you need to remove the quotes and escape the spaces:



                $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
                0






                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered Nov 21 '18 at 20:21









                Jesse_bJesse_b

                12.1k23064




                12.1k23064

























                    2














                    In bash, the canonical answer is: use a var



                    $ reg='^ [0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?


                    That works for spaces, backslash, and many other things:



                    $ reg='^ 123 $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    If you want to write it as a literal you need to play (carefully) with quoting.

                    You need to quote the parts that need to be literal and leave the parts that should be interpreted as a regex un-quoted. The space needs quoting. Here using backslah:



                    $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
                    0


                    And here using double quotes:



                    $ [[ ' 123 ' =~ ^" "[0-9]+" "$ ]]; echo $?


                    But that "quoting" gets real messy with things like a backslash:



                    $ [[ ' 123 ' =~ ^" "[0-9]+"\"[0-9]+" "$ ]]; echo $?
                    0


                    Simpler, safer:



                    $ reg='^ [0-9]+\[0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    And, no, you don't need a subshell to do that (the parenthesis in your question).



                    $ reg='^ [0-9]+\[0-9]+ $'; [[ ' 123 ' =~ $reg ]]; echo $?




                    Yes, that may seem annoying.
                    And, yes, the command you presented happens to work in zsh:



                    $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
                    0


                    But quoting is always a problem (in any shell), what should happen to backslash ?:



                    % [[ ' 123 ' =~ ' 123 ' ]]; echo $?
                    zsh: failed to compile regex: Invalid back reference
                    1


                    % [[ ' 123 ' =~ '^ [0-9]+\3 $' ]]; echo $?
                    0


                    Double it !. It is not exactly: literal.






                    share|improve this answer


























                    • I know that the subshell is not needed for the match; I use subshells to avoid polluting my current shell's namespace with a variable that I need to use only once.

                      – kjo
                      Nov 21 '18 at 21:08













                    • An unset regexp is always faster than a subshell. @kjo

                      – Isaac
                      Nov 21 '18 at 23:12











                    • Maybe, but I don't care about the few milliseconds here and there. The subshell takes fewer keystrokes, it is robust to any number of changes I may do in the subshell (not just the setting of one variable), and there's no chance I will forget to run unset afterwards.

                      – kjo
                      Nov 22 '18 at 16:54













                    • Sure, why fight the waste if it is easier?. Why reduce the use of plastics until we get a plastic planet? @kjo

                      – Isaac
                      Nov 23 '18 at 0:49
















                    2














                    In bash, the canonical answer is: use a var



                    $ reg='^ [0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?


                    That works for spaces, backslash, and many other things:



                    $ reg='^ 123 $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    If you want to write it as a literal you need to play (carefully) with quoting.

                    You need to quote the parts that need to be literal and leave the parts that should be interpreted as a regex un-quoted. The space needs quoting. Here using backslah:



                    $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
                    0


                    And here using double quotes:



                    $ [[ ' 123 ' =~ ^" "[0-9]+" "$ ]]; echo $?


                    But that "quoting" gets real messy with things like a backslash:



                    $ [[ ' 123 ' =~ ^" "[0-9]+"\"[0-9]+" "$ ]]; echo $?
                    0


                    Simpler, safer:



                    $ reg='^ [0-9]+\[0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    And, no, you don't need a subshell to do that (the parenthesis in your question).



                    $ reg='^ [0-9]+\[0-9]+ $'; [[ ' 123 ' =~ $reg ]]; echo $?




                    Yes, that may seem annoying.
                    And, yes, the command you presented happens to work in zsh:



                    $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
                    0


                    But quoting is always a problem (in any shell), what should happen to backslash ?:



                    % [[ ' 123 ' =~ ' 123 ' ]]; echo $?
                    zsh: failed to compile regex: Invalid back reference
                    1


                    % [[ ' 123 ' =~ '^ [0-9]+\3 $' ]]; echo $?
                    0


                    Double it !. It is not exactly: literal.






                    share|improve this answer


























                    • I know that the subshell is not needed for the match; I use subshells to avoid polluting my current shell's namespace with a variable that I need to use only once.

                      – kjo
                      Nov 21 '18 at 21:08













                    • An unset regexp is always faster than a subshell. @kjo

                      – Isaac
                      Nov 21 '18 at 23:12











                    • Maybe, but I don't care about the few milliseconds here and there. The subshell takes fewer keystrokes, it is robust to any number of changes I may do in the subshell (not just the setting of one variable), and there's no chance I will forget to run unset afterwards.

                      – kjo
                      Nov 22 '18 at 16:54













                    • Sure, why fight the waste if it is easier?. Why reduce the use of plastics until we get a plastic planet? @kjo

                      – Isaac
                      Nov 23 '18 at 0:49














                    2












                    2








                    2







                    In bash, the canonical answer is: use a var



                    $ reg='^ [0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?


                    That works for spaces, backslash, and many other things:



                    $ reg='^ 123 $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    If you want to write it as a literal you need to play (carefully) with quoting.

                    You need to quote the parts that need to be literal and leave the parts that should be interpreted as a regex un-quoted. The space needs quoting. Here using backslah:



                    $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
                    0


                    And here using double quotes:



                    $ [[ ' 123 ' =~ ^" "[0-9]+" "$ ]]; echo $?


                    But that "quoting" gets real messy with things like a backslash:



                    $ [[ ' 123 ' =~ ^" "[0-9]+"\"[0-9]+" "$ ]]; echo $?
                    0


                    Simpler, safer:



                    $ reg='^ [0-9]+\[0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    And, no, you don't need a subshell to do that (the parenthesis in your question).



                    $ reg='^ [0-9]+\[0-9]+ $'; [[ ' 123 ' =~ $reg ]]; echo $?




                    Yes, that may seem annoying.
                    And, yes, the command you presented happens to work in zsh:



                    $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
                    0


                    But quoting is always a problem (in any shell), what should happen to backslash ?:



                    % [[ ' 123 ' =~ ' 123 ' ]]; echo $?
                    zsh: failed to compile regex: Invalid back reference
                    1


                    % [[ ' 123 ' =~ '^ [0-9]+\3 $' ]]; echo $?
                    0


                    Double it !. It is not exactly: literal.






                    share|improve this answer















                    In bash, the canonical answer is: use a var



                    $ reg='^ [0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?


                    That works for spaces, backslash, and many other things:



                    $ reg='^ 123 $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    If you want to write it as a literal you need to play (carefully) with quoting.

                    You need to quote the parts that need to be literal and leave the parts that should be interpreted as a regex un-quoted. The space needs quoting. Here using backslah:



                    $ [[ ' 123 ' =~ ^ [0-9]+ $ ]]; echo $?
                    0


                    And here using double quotes:



                    $ [[ ' 123 ' =~ ^" "[0-9]+" "$ ]]; echo $?


                    But that "quoting" gets real messy with things like a backslash:



                    $ [[ ' 123 ' =~ ^" "[0-9]+"\"[0-9]+" "$ ]]; echo $?
                    0


                    Simpler, safer:



                    $ reg='^ [0-9]+\[0-9]+ $'
                    $ [[ ' 123 ' =~ $reg ]]; echo $?
                    0


                    And, no, you don't need a subshell to do that (the parenthesis in your question).



                    $ reg='^ [0-9]+\[0-9]+ $'; [[ ' 123 ' =~ $reg ]]; echo $?




                    Yes, that may seem annoying.
                    And, yes, the command you presented happens to work in zsh:



                    $ [[ ' 123 ' =~ '^ [0-9]+ $' ]]; echo $?
                    0


                    But quoting is always a problem (in any shell), what should happen to backslash ?:



                    % [[ ' 123 ' =~ ' 123 ' ]]; echo $?
                    zsh: failed to compile regex: Invalid back reference
                    1


                    % [[ ' 123 ' =~ '^ [0-9]+\3 $' ]]; echo $?
                    0


                    Double it !. It is not exactly: literal.







                    share|improve this answer














                    share|improve this answer



                    share|improve this answer








                    edited Nov 21 '18 at 20:40

























                    answered Nov 21 '18 at 20:23









                    IsaacIsaac

                    11.6k11652




                    11.6k11652













                    • I know that the subshell is not needed for the match; I use subshells to avoid polluting my current shell's namespace with a variable that I need to use only once.

                      – kjo
                      Nov 21 '18 at 21:08













                    • An unset regexp is always faster than a subshell. @kjo

                      – Isaac
                      Nov 21 '18 at 23:12











                    • Maybe, but I don't care about the few milliseconds here and there. The subshell takes fewer keystrokes, it is robust to any number of changes I may do in the subshell (not just the setting of one variable), and there's no chance I will forget to run unset afterwards.

                      – kjo
                      Nov 22 '18 at 16:54













                    • Sure, why fight the waste if it is easier?. Why reduce the use of plastics until we get a plastic planet? @kjo

                      – Isaac
                      Nov 23 '18 at 0:49



















                    • I know that the subshell is not needed for the match; I use subshells to avoid polluting my current shell's namespace with a variable that I need to use only once.

                      – kjo
                      Nov 21 '18 at 21:08













                    • An unset regexp is always faster than a subshell. @kjo

                      – Isaac
                      Nov 21 '18 at 23:12











                    • Maybe, but I don't care about the few milliseconds here and there. The subshell takes fewer keystrokes, it is robust to any number of changes I may do in the subshell (not just the setting of one variable), and there's no chance I will forget to run unset afterwards.

                      – kjo
                      Nov 22 '18 at 16:54













                    • Sure, why fight the waste if it is easier?. Why reduce the use of plastics until we get a plastic planet? @kjo

                      – Isaac
                      Nov 23 '18 at 0:49

















                    I know that the subshell is not needed for the match; I use subshells to avoid polluting my current shell's namespace with a variable that I need to use only once.

                    – kjo
                    Nov 21 '18 at 21:08







                    I know that the subshell is not needed for the match; I use subshells to avoid polluting my current shell's namespace with a variable that I need to use only once.

                    – kjo
                    Nov 21 '18 at 21:08















                    An unset regexp is always faster than a subshell. @kjo

                    – Isaac
                    Nov 21 '18 at 23:12





                    An unset regexp is always faster than a subshell. @kjo

                    – Isaac
                    Nov 21 '18 at 23:12













                    Maybe, but I don't care about the few milliseconds here and there. The subshell takes fewer keystrokes, it is robust to any number of changes I may do in the subshell (not just the setting of one variable), and there's no chance I will forget to run unset afterwards.

                    – kjo
                    Nov 22 '18 at 16:54







                    Maybe, but I don't care about the few milliseconds here and there. The subshell takes fewer keystrokes, it is robust to any number of changes I may do in the subshell (not just the setting of one variable), and there's no chance I will forget to run unset afterwards.

                    – kjo
                    Nov 22 '18 at 16:54















                    Sure, why fight the waste if it is easier?. Why reduce the use of plastics until we get a plastic planet? @kjo

                    – Isaac
                    Nov 23 '18 at 0:49





                    Sure, why fight the waste if it is easier?. Why reduce the use of plastics until we get a plastic planet? @kjo

                    – Isaac
                    Nov 23 '18 at 0:49


















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