Combining regular expressions in bash script

Question

I am trying to compare a string with the $ character, followed by either s or t and a range of numbers. but the range is different for each letter. for example if it is an s it can be followed by numbers between 0-7 and if it is a t it can be followed by numbers between 0-9.

What I managed to write a part of the if statement to compare it, but I couldn't figure out how to include different ranges for each letter

script:

#!/bin/bash
input="test1.txt"
check(){
    while read -r line; do
      a=( $line )
      for i in "${a[@]:1}"; do
         if [[ "$i" == \$[st]* ]]; then
             echo "$i"
         fi
      done
    done < "$input"
}
check

Instead of using * I want to specify for s that it can only be followed by numbers between 0-7 and t can only be followed by numbers 0-9. I tried using this:

if [[ "$i" == \$(s[0-7]*|t[0-9]*) ]]; then

but I got this error:

./test.sh: line 9: syntax error in conditional expression: unexpected token `('                                         ./test.sh: line 9: syntax error near `\$(s'                                                                             ./test.sh: line 9: `if [[ "$i" == \$(s[0-7]*|t[0-9]*) ]]; then'

Note that a=( $line ) is a dangerous practice for the reasons described in BashPitfalls #50 (while the pitfall refers to command substitutions, unquoted parameter expansions behave identically for purposes thereof) -- better to make it while read -r -a a; do and have read split the words of your line into array elements itself. — Charles Duffy
– Charles Duffy, Commented Mar 28, 2019 at 15:02

oguz ismail · Accepted Answer · 2019-04-26 20:38:38Z

7

=~ is used for regex matching, not ==. Correct that and use a pipe (|) in your regex, which means OR.

if [[ $i =~ \$(s[0-7]*|t[0-9]*) ]]

edited Apr 26, 2019 at 20:38

answered Mar 28, 2019 at 14:52

oguz ismail

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3 Comments

glenn jackman Over a year ago

@David, also note that =~ is a bash-specific feature, so you cannot run your code like sh ./myscript

Charles Duffy Over a year ago

Personally, I would use [$] rather than \$ -- it's a bit more robust across different quoting contexts/styles -- but that's a trivial quibble; this is a good solution either way.

chepner Over a year ago

If you really want to use ==, you can use the extended pattern [$]@(s*([0-7])|t*([0-9])) instead.

Dominique · Accepted Answer · 2019-03-28 15:07:43Z

1

Why are you writing a script with a loop for this? It can simply be done using a grep, as you can see in this example:

Prompt>cat test1.txt
s1
s8
t1
t8
Prompt> grep -E "s[0-7]|t[0-9]" test1.txt
s1
t1
t8

Explanation:

s[0-7] means the letter "s", followed by any digit from 0 to 7.
t[0-9] means the letter "t", followed by any digit from 0 to 9.

grep -E means "Extended regular expression".

A|B in the regular expression means The expression should contain A OR B (which is exactly the "extended" part of the regular expression).

answered Mar 28, 2019 at 15:07

Dominique

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1 Comment

double-beep Over a year ago

FYI, your post was incorrectly flagged as "low quality". It has been handled accordingly. From Review.

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