Bash - correct way to escape dollar in regex

Question

What is the correct way to escape a dollar sign in a bash regex? I am trying to test whether a string begins with a dollar sign. Here is my code, in which I double escape the dollar within my double quotes expression:

echo -e "AB1\nAB2\n\$EXTERNAL_REF\nAB3" | while read value;
do
    if [[ ! $value =~ "^\\$" ]];
    then
            echo $value
    else
            echo "Variable found: $value"
    fi
done

This does what I want for one box which has:

GNU bash, version 3.2.25(1)-release (x86_64-redhat-linux-gnu)

And the verbose output shows

+ [[ ! $EXTERNAL_REF =~ ^\$ ]]
+ echo 'Variable found: $EXTERNAL_REF'

However, on another box which uses

GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)

The comparison is expanded as follows

+ [[ ! $EXTERNAL_REF =~ \^\\\$ ]]
+ echo '$EXTERNAL_REF'

Is there a standard/better way to do this that will work across all implementations?

Many thanks

score 3 · Accepted Answer · answered Dec 27 '12 at 10:14

3

Why do you use a regular expression here? A glob is enough:

#!/bin/bash

while read value; do
   if [[ "$value" != \$* ]]; then
       echo "$value"
   else
       echo "Variable found: $value"
   fi
done < <(printf "%s\n" "AB1" "AB2" '$EXTERNAL_REF' "AB3")

Works here with shopt -s compat32.

answered Dec 27 '12 at 10:14

gniourf_gniourf

44,650
9
93
104

Perfect, that's a nice workaround and does the trick with both implementations. Thanks – user1198484 Dec 27 '12 at 11:35

score 3 · Answer 2 · answered Nov 14 '16 at 13:05

3

The regex doesn't need any quotes at all. This should work:

if [[ ! $value =~ ^\$ ]];

answered Nov 14 '16 at 13:05

Ian Mackinnon

13,381
13
51
67

MajorBlackViking · Answer 3 · 2012-12-27T10:22:14.903

1

I would replace the double quotes with single quotes and remove a single \ and have the changes as below

$value =~ "^\\$"

can also be used as

$value =~ '^\$'

edited Dec 27 '12 at 10:22

answered Dec 27 '12 at 10:11

MajorBlackViking

100
1
8

As above, this doesn't seem to work for me on 4.1.2. If anyone knows why I would be intrigued to know – user1198484 Dec 27 '12 at 11:36

arwild01 · Answer 4 · 2014-07-02T18:36:44.060

I never found the solution either, but for my purposes, I settled on the following workaround:

if [[ "$value" =~ ^(.)[[:alpha:]_][[:alnum:]_]+\\b && ${BASH_REMATCH[1]} == '$' ]]; then
    echo "Variable found: $value"
else
    echo "$value"
fi

Rather than trying to "quote" the dollar-sign, I instead match everything around it and I capture the character where the dollar-sign should be to do a direct-string comparison on. A bit of a kludge, but it works.

Alternatively, I've taken to using variables, but just for the backslash character (I don't like storing the entire regex in a variable because I find it confusing for the regex to not appear in the context where it's used):

bs="\\"
string="test\$test"
if [[ "$string" =~ $bs$ ]]; then
    echo "output \"$BASH_REMATCH\""
fi

Bash - correct way to escape dollar in regex

4 Answers4