I want to match an input string (contained in the variable $1) with a regex representing the date formats MM/DD/YYYY and MM-DD-YYYY.
REGEX_DATE="^\d{2}[\/\-]\d{2}[\/\-]\d{4}$"
echo "$1" | grep -q $REGEX_DATE
echo $?
The echo $? returns the error code 1 no matter the input string.
To complement the existing helpful answers:
Using Bash's own regex-matching operator, =~, is a faster alternative in this case, given that you're only matching a single value already stored in a variable:
set -- '12-34-5678' # set $1 to sample value
kREGEX_DATE='^[0-9]{2}[-/][0-9]{2}[-/][0-9]{4}$' # note use of [0-9] to avoid \d
[[ $1 =~ $kREGEX_DATE ]]
echo $? # 0 with the sample value, i.e., a successful match
Note that =~ even allows you to define capture groups (parenthesized subexpressions) whose matches you can later access through Bash's special ${BASH_REMATCH[@]} array variable.
Portability caveat:
While =~ supports EREs (extended regular expressions), it also supports the host platform's specific extensions - it's a rare case of Bash's behavior being platform-dependent; examples:
\d to match a digit is supported on macOS, but not on Linux - use [0-9]
\< /\> and \b (word-boundary assertions) are supported on Linux, but not on macOS, where you must use [[:<:]] / [[:>:]] - none these are POSIX-compliant
Backreferences (e.g. \1) work on Linux, but not on macOS (per POSIX, they're only supported in basic regexes (BREs)).
If your scripts are designed to run Linux only, you can use a backreference to make matching more robust, by capturing the specific character used as the first separator in a capture group ((...)) and referring to it later with \1 to ensure that the same separator is matched:Thanks, ibonyun
kREGEX_DATE='^[0-9]{2}([-/])[0-9]{2}\1[0-9]{4}$'
To remain portable (in the context of Bash), stick to the POSIX ERE specification.
Further notes:
$kREGEX_DATE is used unquoted, which is necessary for the regex to be recognized as such (quoted parts would be treated as literals).
While not always necessary, it is advisable to store the regex in a variable first, because Bash has trouble with regex literals containing \.
\< is supported to match word boundaries, [[ 3 =~ \<3 ]] && echo yes doesn't work, but re='\<3'; [[ 3 =~ $re ]] && echo yes does.I've changed variable name REGEX_DATE to kREGEX_DATE (k signaling a (conceptual) constant), so as to ensure that the name isn't an all-uppercase name, because all-uppercase variable names should be avoided to prevent conflicts with special environment and shell variables.
^[0-9]{2}([-/])[0-9]{2}\1[0-9]{4}$ instead. It's the same except that the 1st delimiter is inside a capturing group ([-/]) and the 2nd delimiter is a back-reference to whatever that capturing group matches \1. That way you're guaranteed to only match strings where the delimiters are the same which will avoid false positives for strings like "12-34/5678" that coincidently look similar to dates but could mean something completely different.
I think this is what you want:
REGEX_DATE='^\d{2}[/-]\d{2}[/-]\d{4}$'
echo "$1" | grep -P -q $REGEX_DATE
echo $?
I've used the -P switch to get perl regex.
-P is not guaranteed to be supported in all distros. so if portability is a concern, you'll want to avoid it.
the problem is you're trying to use regex features not supported by grep. namely, your \d won't work. use this instead:
REGEX_DATE="^[[:digit:]]{2}[-/][[:digit:]]{2}[-/][[:digit:]]{4}$"
echo "$1" | grep -qE "${REGEX_DATE}"
echo $?
you need the -E flag to get ERE in order to use {#} style.
\ -escaped the { and } instances, this particular regex would have worked without -E, as a BRE (basic regex) as well; as a non-portable aside: BSD/OSX grep - unlike GNU grep - actually does support \d.-E over `` everywhere as it makes the code much more readable, and it's in POSIX.Below is example shell script, which we used for pre-commit hook to enforce branch name
#!/bin/bash
BRANCH_NAME=$(git rev-parse --abbrev-ref HEAD)
BRANCH_REGEX='^(feature|hotfix|chore)/MYAPP-[0-9]+(_|-).*$'
if ! [[ $BRANCH_NAME =~ $BRANCH_REGEX ]]; then
echo "Error: Invalid branch name $BRANCH_NAME."
echo "You can run below command to rename branch"
echo "git branch -m old_branch new_branch"
fi
$?reports on the first command in the pipe chain, which is echo - the echo will obviously succeed, so you get a1exit code. trygrep $pattern <<< $1instead.$?is the last exit status in the pipeline