Bash Replace String in a variable using regex

Question

I am trying to replace a string in a variable using regex. However as soon as I use regex it's not working.

Source String:

The number could be anything, any valid integer.

message='"autotermination_minutes": 15,'

Expected String

   message='"autotermination_minutes": 30,'

Following is working (without regex)

echo ${message/'"autotermination_minutes": 15'/'"autotermination_minutes": 30'}

Following doesn't work (with Regex)

echo ${message/'"autotermination_minutes": '[0-9]/'"autotermination_minutes": 30'}

it shows, following output

"autotermination_minutes": 305,

Answer 1

[0-9] only matches a single digit.

Use [0-9][0-9] to match two:

➜ echo ${message/'"autotermination_minutes": '[0-9][0-9]/'"autotermination_minutes": 30'}
"autotermination_minutes": 30,

Note that Bash parameter substitutions are very limited in functionality. What you're doing here doesn't even have anything to do with regular expressions—these patterns are just globs. So, instead of a [0-9]+ that you'd expect from a regex, you need to use +([0-9]) to match one or more digits:

➜ shopt -s extglob
➜ echo ${message/'"autotermination_minutes": '+([0-9])/'"autotermination_minutes": 30'}
"autotermination_minutes": 15,

See also here .

But you really should be using sed or perl :

➜ echo "$message" | sed -E 's/[0-9]+/30/'
"autotermination_minutes": 30,

Or, if your input is JSON, you really should be using a JSON parser and read/write the file properly rather than using regular expressions.

Answer 2

In pure bash, there is no way to capture a value from original string and have it back in replacement using a back-reference.

You may use a simple sed here with a capture group and back-reference:

message='"autotermination_minutes": 15,'
sed -E 's/("autotermination_minutes": *)[0-9]+/\130/' <<< "$message"

"autotermination_minutes": 30,

Answer 3

How about a bash-only solution:

message='"autotermination_minutes": 15,'
pattern='(.*autotermination_minutes":[[:blank:]]*)([[:digit:]]+)(.*)'
replacement='30'

if [[ $message =~ $pattern ]]; then
    echo "${BASH_REMATCH[1]}${replacement}${BASH_REMATCH[3]}"
else
    echo "$message"
fi

Note that it is less efficient in both code size and execution time than properly designed sed or awk solution.