How do I use regular expressions in bash scripts?

Question

I want to check if a variable has a valid year using a regular expression. Reading the bash manual I understand I could use the operator =~

Looking at the example below, I would expect to see "not OK" but I see "OK". What am I doing wrong?

i="test"
if [ $i=~"200[78]" ]
then
  echo "OK"
else
  echo "not OK"
fi

Note this was failing because of the lack of spaces around =~. — fedorqui
– fedorqui, Commented Jun 20, 2016 at 9:24

Community · Accepted Answer · 2020-06-20 09:12:55Z

118

It was changed between 3.1 and 3.2:

This is a terse description of the new features added to bash-3.2 since the release of bash-3.1.

Quoting the string argument to the [[ command's =~ operator now forces string matching, as with the other pattern-matching operators.

So use it without the quotes thus:

i="test"
if [[ $i =~ 200[78] ]] ; then
    echo "OK"
else
    echo "not OK"
fi

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Nov 20, 2008 at 10:45

paxdiablo

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2 Comments

Alderath Over a year ago

How do i handle the situation when the regex contains spaces if I cannot quote? If the regex is e.g. a +b it will report a syntax error...

blinry Over a year ago

@Alderath: Use a\ \+b to escape the space and the plus character.

michiel · Accepted Answer · 2010-02-24 00:54:58Z

8

You need spaces around the operator =~

i="test"
if [[ $i =~ "200[78]" ]];
then
  echo "OK"
else
  echo "not OK"
fi

answered Feb 24, 2010 at 0:54

michiel

971 silver badge1 bronze badge

1 Comment

Marcel Stimberg Over a year ago

paxdiablo's answer is right, adding spaces here does not help (you now also get "not OK" for 2008, the only string that gets matched is literally "200[78]").

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