Regex matching in a Bash if statement

Question

What did I do wrong here   Trying to match any string that contains spaces  lowercase  uppercase  or numbers  Special characters would be nice too  but I think that requires escaping certain characters   TEST  THIS is a TEST title with some numbers 12345 and special char   amp        if      TEST       a-zA-Z0-9        then BLAH  fi   This obviously only tests for upper  lower  numbers  and spaces  Doesn t work though     UPDATE    I guess I should have been more specific  Here is the actual real line of code   if      TITLE       a-zA-Z0-9        then RETURN  FAIL   amp  amp  ERROR  ERROR  Title can only contain upper and lowercase letters  numbers  and spaces    fi     UPDATE      anm sh  line 265  syntax error in conditional expression   anm sh  line 265  syntax error near   amp        anm sh  line 265     if        TITLE      a-zA-Z0-9      amp         then RETURN  FAIL   amp  amp  ERROR  ERROR  Title can only contain upper and lowercase letters  numbers  and spaces    return  fi

User · Answer

In case someone wanted an example using variables        bin bash    Only continue for  develop  or  release    branches BRANCH REGEX    develop  release       if     BRANCH     BRANCH REGEX     then     echo  BRANCH   BRANCH  matches BRANCH REGEX   BRANCH REGEX   else     echo  BRANCH   BRANCH  DOES NOT MATCH BRANCH REGEX   BRANCH REGEX   fi

User · Answer

Or you might be looking at this question because you happened to make a silly typo like I did and have the    reversed to

User · Answer

I d prefer to use   punct   for that  Also  a-zA-Z09-9 could be just   alnum         TEST        alnum    blank    punct

User · Answer

There are a couple of important things to know about bash s       construction  The first      Word splitting and pathname expansion are not performed on the words between the    and     tilde expansion  parameter and variable expansion  arithmetic expansion  command substitution  process substitution  and quote removal are performed    The second thing      An additional binary operator            is available     the string to the right of the operator is considered an extended regular expression and matched accordingly    Any part of the pattern may be quoted to force it to be matched as a string    Consequently   v on either side of the    will be expanded to the value of that variable  but the result will not be word-split or pathname-expanded  In other words  it s perfectly safe to leave variable expansions unquoted on the left-hand side  but you need to know that variable expansions will happen on the right-hand side   So if you write      x      0-9a-zA-Z      the  0 inside the regex on the right will be expanded before the regex is interpreted  which will probably cause the regex to fail to compile  unless the expansion of  0 ends with a digit or punctuation symbol whose ascii value is less than a digit   If you quote the right-hand side like-so     x       0-9a-zA-Z       then the right-hand side will be treated as an ordinary string  not a regex  and  0 will still be expanded   What you really want in this case is     x       0-9a-zA-Z      Similarly  the expression between the    and    is split into words before the regex is interpreted  So spaces in the regex need to be escaped or quoted  If you wanted to match letters  digits or spaces you could use      x     0-9a-zA-Z        Other characters similarly need to be escaped  like    which would start a comment if not quoted  Of course  you can put the pattern into a variable   pat   0-9a-zA-Z    if     x     pat     then       For regexes which contain lots of characters which would need to be escaped or quoted to pass through bash s lexer  many people prefer this style  But beware  In this case  you cannot quote the variable expansion     This doesn t work  if     x      pat      then       Finally  I think what you are trying to do is to verify that the variable only contains valid characters  The easiest way to do this check is to make sure that it does not contain an invalid character  In other words  an expression like this   valid  0-9a-zA-Z    amp      add almost whatever else you want to allow to the list if       x       valid      then         negates the test  turning it into a  does not match  operator  and a        regex character class means  any character other than          The combination of parameter expansion and regex operators can make bash regular expression syntax  almost readable   but there are still some gotchas   Aren t there always   One is that you could not put   into  valid  even if  valid were quoted  except at the very beginning   That s a Posix regex rule  if you want to include   in a character class  it needs to go at the beginning  - can go at the beginning or the end  so if you need both   and -  you need to start with   and end with -  leading to the regex  I know what I m doing  emoticon     -

[regex] Regex matching in a Bash if statement

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