checking if parameter is regex

[lackawaxen]

hi, all.

i have a ksh script that can take a regular expression when invoked:

./script_name arg1 <arg2> <arg3> <regex>

the regex is optional. within my script, then, i need to check if the regex was added. this is what i'm thinking:

for i in $*
  if [ $i begins with any of these '[][\\.*$^]/\\&' ]
    do this
  fi
done

but i don't know how the syntax for doing this. i would appreciate any help you can provide.

thanks.

[burschik]

Shouldn't you be checking the number of arguments? And since fixed strings are also valid regular expressions, there is no way to determine whether a particular string is supposed to be a regular expression, or not.

[wje_lf]

Shouldn't you be checking the number of arguments?

burschik, you're quite possibly correct. But I'm guessing that in his example, the <> stands for an optional argument. (lackawaxen, it would have been a kindness if you had spelled that out for us, so we wouldn't be spinning our wheels here figuring out what you meant.) So the second and third arguments could be missing and he could still have a regular expression.

since fixed strings are also valid regular expressions, there is no way to determine whether a particular string is supposed to be a regular expression, or not.

True. For example, this:

fred.txt

or even this:

fred

is a valid regular expression.

The safest thing to do is to rework what he requires as syntax in his script arguments. Perhaps instead of this:

./script_name arg1 <arg2> <arg3> <regex>

He should have this:

./script_name arg1 <arg2> <arg3> <-e regex>

That is, if the caller of his script wants to add a regular expression, then the caller needs to add not one, but two arguments, and the first of these two is -e.

[lackawaxen]

Thank you both for your responses. I apologize for not being more clear. wfe_le is correct: I used <> to indicate optional. Sorry about that.

I don't think it matters if the regex is fred or ^f. I am only interested in those that contain a special character. In my original post, I said that I want to know if an argument "begins" with a special character; but I should have said "contain" for cases such as f$.

In any event, I suppose what I really want to know is how to express stuff like this in ksh:


If parameter begins with X
If parameter ends with X
If parameter contains X


Thanks.

[burschik]

burschik, you're quite possibly correct. But I'm guessing that in his example, the <> stands for an optional argument.

I had guessed as much. However, if you have several optional arguments and no switches, it is generally the case that the number of arguments determines the type of the arguments. "seq" is an excellent example of this usage pattern.

[burschik]

In any event, I suppose what I really want to know is how to express stuff like this in ksh:


If parameter begins with X
If parameter ends with X
If parameter contains X


Thanks.

For the first two, you can use parameter expansion. For the last item, you will probably need to call "grep", or "sed", or something like that.

[drl]

Hi.

       string == pattern
              True, if string matches pattern.  Any part  of  pattern  can  be
              quoted to cause it to be matched as a string.  With a successful
              match to a pattern, the .sh.match array  variable  will  contain
              the match and sub-pattern matches.
       string != pattern
              True, if string does not match pattern.  When the string matches
              the pattern the .sh.match array variable will contain the  match
              and sub-pattern matches.
       string =~ ere
              True  if  string  matches  the  pattern  ~(E)ere where ere is an
              extended regular expression.

-- excerpt from man ksh in section describing conditions allowable with [[ compound command for ksh93

I suggest reading the man page and then writing a few scripts to explore the possibilities ... cheers, drl

[burschik]

Nice, but my ksh can't do that.

[drl]

Hi, burschik.

Nice, but my ksh can't do that.

There is an alternative:

              When the == and != operators are used, the string to  the  right
              of the operator is considered a pattern and matched according to
              the rules described below under Pattern Matching.  If the  shell
              option  nocasematch  is  enabled, the match is performed without
              regard to the case of alphabetic characters.  The  return  value
              is  0 if the string matches (==) or does not match (!=) the pat-
              tern, and 1 otherwise.  Any part of the pattern may be quoted to
              force it to be matched as a string.

              An  additional  binary operator, =~, is available, with the same
              precedence as == and !=.  When it is used,  the  string  to  the
              right  of the operator is considered an extended regular expres-
              sion and matched accordingly (as in regex(3)).

-- excerpt from man bash for GNU bash 3.2.39

which may not help the OP, but might be useful in other situations ... cheers, drl