How do I use a regex in a shell script

Question

I am trying to match a string with a regex in a shell script  This string is a parameter of the script    1   and it is a date  MM DD YYYY  The regex I m trying to use is       d 2     -  d 2     -  d 4     It seems to work  I tried it on several regex tests websites   My shell code is     REGEX DATE    d 2     -  d 2     -  d 4       echo   1    grep -q  REGEX DATE echo      The  echo     returns 1 no matter is the string I m putting in parameter   Do you guys have an idea    Thanks

User · Answer

the problem is you re trying to use regex features not supported by grep   namely  your  d won t work   use this instead   REGEX DATE      digit    2  -     digit    2  -     digit    4    echo   1    grep -qE    REGEX DATE   echo      you need the -E flag to get ERE in order to use     style

User · Answer

To complement the existing helpful answers   Using Bash s own regex-matching operator      is a faster alternative in this case  given that you re only matching a single value already stored in a variable   set --  12-34-5678    set  1 to sample value  kREGEX DATE    0-9  2  -   0-9  2  -   0-9  4      note use of  0-9  to avoid  d     1     kREGEX DATE    echo      0 with the sample value  i e   a successful match   Note  however  that the caveat re using flavor-specific regex constructs such as  d equally applies  While    supports EREs  extended regular expressions   it also supports the host platform s specific extension - it s a rare case of Bash s behavior being platform-dependent   To remain portable  in the context of Bash   stick to the POSIX ERE specification   Note that    even allows you to define capture groups  parenthesized subexpressions  whose matches you can later access through Bash s special   BASH REMATCH     array variable   Further notes     kREGEX DATE is used unquoted  which is necessary for the regex to be recognized as such  quoted parts would be treated as literals   While not always necessary  it is advisable to store the regex in a variable first  because Bash has trouble with regex literals containing      E g   on Linux  where   lt  is supported to match word boundaries     3      lt 3     amp  amp  echo yes doesn t work  but re    lt 3      3     re     amp  amp  echo yes does   I ve changed variable name REGEX DATE to kREGEX DATE  k signaling a  conceptual  constant   so as to ensure that the name isn t an all-uppercase name  because all-uppercase variable names should be avoided to prevent conflicts with special environment and shell variables

User · Answer

I think this is what you want   REGEX DATE    d 2   -  d 2   -  d 4     echo   1    grep -P -q  REGEX DATE echo      I ve used the -P switch to get perl regex

[regex] How do I use a regex in a shell script?

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