Check if a string matches a regex in Bash script

Question

One of the arguments that my script receives is a date in the following format  yyyymmdd   I want to check if I get a valid date as an input   How can I do this  I am trying to use a regex like   0-9    8

User · Answer

In bash version 3 you can use the      operator   if      date       0-9  8       then     echo  Valid date  else     echo  Invalid date  fi   Reference  http   tldp org LDP abs html bashver3 html REGEXMATCHREF     NOTE  The quoting in the matching operator within the double brackets         is no longer necessary as of Bash version 3 2

User · Answer

Where the usage of a regex can be helpful to determine if the character sequence of a date is correct  it cannot be used easily to determine if the date is valid  The following examples will pass the regular expression  but are all invalid dates  20180231  20190229  20190431  So if you want to validate if your date string  let s call it datestr  is in the correct format  it is best to parse it with date and ask date to convert the string to the correct format  If both strings are identical  you have a valid format and valid date    if      datestr       date -d   datestr     Y m d  2 gt  dev null      then      echo  Valid date  else      echo  Invalid date  fi

User · Answer

I would use expr match instead of      expr match   date    0-9   8     gt  dev null  amp  amp  echo yes   This is better than the currently accepted answer of using    because    will also match empty strings  which IMHO it shouldn t  Suppose badvar is not defined  then     1234       badvar      echo    gives  incorrectly  0  while expr match  1234    badvar   gt  dev null   echo    gives correct result 1   We have to use  gt  dev null to hide expr match s output value  which is the number of characters matched or 0 if no match found  Note its output value is different from its exit status  The exit status is 0 if there s a match found  or 1 otherwise   Generally  the syntax for expr is   expr match   string    lead    Or   expr   string      lead    where  lead is a regular expression  Its exit status will be true  0  if lead matches the leading slice of string  Is there a name for this    For example expr match  abcdefghi   abc exits true  but expr match  abcdefghi   bcd  exits false   Credit to  Carlo Wood for pointing out this

User · Answer

You can use the test construct         along with the regular expression match operator      to check if a string matches a regex pattern  For your specific case  you can write      date      0-9  8       amp  amp  echo  quot yes quot   Or more a accurate test      date      0-9  4  0 1-9  1 0-2   0 1-9   1-2  0-9  3 0-1        amp  amp  echo  quot yes quot                                                                                                                                                                                                                                                                                --year--   --month--           --day--                                   either 01   09      either 01  09     end of line   start of line            or 10 11 12         or 10  29                                                or 30  31  That is  you can define a regex in Bash matching the format you want  This way you can do      date     regex      amp  amp  echo  quot matched quot     echo  quot did not match quot   where commands after  amp  amp  are executed if the test is successful  and commands after    are executed if the test is unsuccessful  Note this is based on the solution by Aleks-Daniel Jakimenko in User input date format verification in bash   In other shells you can use grep  If your shell is POSIX compliant  do  echo  quot  date quot    grep -Eq   regex    amp  amp  echo  quot matched quot     echo  quot did not match quot   In fish  which is not POSIX-compliant  you can do echo  quot  date quot    grep -Eq  quot  regex   quot   and echo  quot matched quot   or echo  quot did not match quot

User · Answer

A good way to test if a string is a correct date is to use the command date   if date -d    DATE    gt  dev null 2 gt  amp 1 then     do what you need to do with your date else   echo    DATE  incorrect date   gt  amp 2   exit 1 fi   from comment  one can use formatting  if    2017-01-14       date -d  2017-01-14     Y- m- d

[regex] Check if a string matches a regex in Bash script

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