Bash Regex to check if first character of string is a number

Question

I'm writing a script that uses trace route. I'm iterating through each line of the trace route and then through each word (separated by whitespace). However, sometimes the trace route returns a * character, which causes issues when echoing (filenames are output).

I've been fiddling with RegEx and so far I've come up with this:

if [[ $item =~ ^\d ]];

Item is a portion of the trace route.

For each item in a trace route line, I would simply like to check if the first character is equal to a number or not, then continue accordingly.

Answer 1

\d is not supported in POSIX Regular Expressions (used by Bash). You need to replace it with [0-9] like so:

if [[ $item =~ ^[0-9] ]];

Check out this StackOverflow answer

Could also use [:digit:] to make it easier to read:

if [[ $item =~ ^[[:digit:]] ]];

Answer 2

No need to use regex just glob is sufficient:

[[ $item == [0-9]* ]] && echo "it starts with a digit"

You can also use:

[[ $item == [[:digit:]]* ]]