How to compare text with regex which is in variable?

Question

I have the following in input.txt:

EA06\\?\.LFRFLB\\?\..*

I want to know if this pattern match with the following string:

EA06.LFRFLB.SHELLO

Then I coded:

MY_STRING="EA06.LFRFLB.SHELLO"
REGEX=$(cat input.txt) # EA06\\?\.LFRFLB\\?\..*

if [[ "${MY_STRING}" =~ "${REGEX}" ]]; then
  echo "FOUND"
else
 echo "NOT FOUND"
fi

if [[ "${MY_STRING}" =~ EA06\\?\.LFRFLB\\?\..* ]]; then
  echo "FOUND"
else
 echo "NOT FOUND"
fi

Result:

NOT FOUND
FOUND

Whats is going wrong here? Why the first if does not match correctly? What is the best way to solve it?

Shouldn't there be quotes around the output of cat input.txt? — Andrew Barber
– Andrew Barber, Commented May 29, 2014 at 14:39
@Jidder My brain is hurting too much today. hehe... I should keep my head out of this sort of shell scripting stuff for a while ;) — Andrew Barber
– Andrew Barber, Commented May 29, 2014 at 14:42

devnull · Accepted Answer · 2014-05-29 14:41:04Z

6

The problem is that you quoted the variable on the RHS of the binary operator =~ which causes the variable replacement to be treated as literal text and not as a regex. You need to say:

if [[ "${MY_STRING}" =~ ${REGEX} ]]; then

Quoting from the manual:

   An additional binary operator, =~, is available, with the  same  prece‐
   dence  as  ==  and !=.  When it is used, the string to the right of the
   operator is considered  an  extended  regular  expression  and  matched
   accordingly  (as  in  regex(3)).   The  return value is 0 if the string
   matches the pattern, and 1 otherwise.  If  the  regular  expression  is
   syntactically  incorrect,  the conditional expression's return value is
   2.  If the shell option nocasematch is enabled, the match is  performed
   without  regard  to the case of alphabetic characters.  Any part of the
   pattern may be quoted to force it to be  matched  as  a  string.

Note the last line in particular:

Any part of the pattern may be quoted to force it to be matched as a string.

answered May 29, 2014 at 14:41

devnull

124k33 gold badges247 silver badges234 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

chepner Over a year ago

@JohnDoe Note that the usual advice to always quote parameter expansions does not apply inside [[ ... ]], since different rules apply. In particular, such parameter expansions are not subject to filename generation or word splitting

Collectives™ on Stack Overflow

How to compare text with regex which is in variable?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related