Check if a string matches a regex in Bash script

Question

You can use the test construct, [[ ]], along with the regular expression match operator, =~, to check if a string matches a regex pattern.

For your specific case, you can write:

[[ $date =~ ^[0-9]{8}$ ]] && echo "yes"

Or more a accurate test:

[[ $date =~ ^[0-9]{4}(0[1-9]|1[0-2])(0[1-9]|[1-2][0-9]|3[0-1])$ ]] && echo "yes"
#           |\______/\______*______/\______*__________*______/|
#           |   |           |                  |              |
#           |   |           |                  |              |
#           | --year--   --month--           --day--          |
#           |          either 01...09      either 01..09      |
# start of line            or 10,11,12         or 10..29      |
#                                              or 30, 31      |
#                                                        end of line

That is, you can define a regex in Bash matching the format you want. This way you can do:

[[ $date =~ ^regex$ ]] && echo "matched" || echo "did not match"

where commands after && are executed if the test is successful, and commands after || are executed if the test is unsuccessful.

Note this is based on the solution by Aleks-Daniel Jakimenko in User input date format verification in bash.

---

In other shells you can use grep. If your shell is POSIX compliant, do

(echo "$date" | grep -Eq  ^regex$) && echo "matched" || echo "did not match"

In fish, which is not POSIX-compliant, you can do

echo "$date" | grep -Eq "^regex\$"; and echo "matched"; or echo "did not match"

In bash version 3 you can use the '=~' operator:

if [[ "$date" =~ ^[0-9]{8}$ ]]; then
    echo "Valid date"
else
    echo "Invalid date"
fi

Reference: http://tldp.org/LDP/abs/html/bashver3.html#REGEXMATCHREF

NOTE: The quoting in the matching operator within the double brackets, [[ ]], is no longer necessary as of Bash version 3.2

A good way to test if a string is a correct date is to use the command date:

if date -d "${DATE}" >/dev/null 2>&1
then
  # do what you need to do with your date
else
  echo "${DATE} incorrect date" >&2
  exit 1
fi

from comment: one can use formatting

if [ "2017-01-14" == $(date -d "2017-01-14" '+%Y-%m-%d') ] 

In addition to other answers of the =~ Bash operator - Extended Regular Expressions (ERE).

This is the syntax used by awk and egrep (or grep -E),
as well as by Bash's [[ ... =~ ... ]] operator.

For example, a function which supports multiple test provided in multiple arguments:

#!/bin/bash

#-----------#
# Functions #
#-----------#

function RT
{
    declare line_l;

    for line_l in "${@:2}";
    do
        if ! [[ "$line_l" =~ $1 ]];
        then
            return 1;
        fi
    done

    return 0;
}

#-----------#
# Main      #
#-----------#

regex_v='^[0-9]*$';
value_1_v='12345';
value_2_v='67890';

if RT "$regex_v" "$value_1_v" "$value_2_v";
then
    printf 'Valid';
else
    printf 'Invalid';
fi

---

Description

Function RT or Regex Test

# Declare a local variable for a loop.

declare line_l;

# Loop for every argument's value except the first whish is a regex rule

for line_l in "${@:2}";

# Test the value and return a **non-zero** return code if failed

if ! [[ "$line_l" =~ $1 ]];

# Return a **zero** return code - success.

return 0;

Main code

# Define arguments for the function to test

regex_v='^[0-9]*$'; # Regex rule
value_1_v='12345'; # First value
value_2_v='67890'; # Second value

# A statement which runs the function with specified arguments
# and executes `printf 'Valid';` if succeeded, else - `printf 'Invalid';`

if RT "$regex_v" "$value_v";

It should be possible to point at failed argument, for example, by appending a counter in loop and printing its value to stderr.

Related

The quotes around the right-hand side of the =~ operator cause it to become a string, rather than a RegularExpression.

Source