CODEWITHSUNDEEP
regexlinux
jimmy Light
jimmy Light

Reputation: 107

Basic linux regex

I'm learning about Linux command line and have come across a regular expression that has me stumped.

Can someone run through this and explain what its doing please. I know this first part as read will use REPLY as a variable if none is supplied.

read -p "Enter a single item > "

# is input a valid filename?
if [[ $REPLY =~ ^[-[:alnum:]\._]+$ ]]; then 
        echo "'$REPLY' is a valid file name."

Like why is [- in there? Is it checking to see if the start of $REPLY is a dash? Any help would be appreciated.

Upvotes: 0

Views: 86

Answers (1)

Explosion Pills
Explosion Pills

Reputation: 191789

Expression is:

^         -- Start of the string
[         -- Character class start
-         -- A literal dash
[:alnum:] -- POSIX-style alphanumeric (a-z, 0-9)
.         -- A literal period
_         -- A literal underscore
]         -- End character class
+         -- One or more of anything in the character class
$         -- End of the string

The - is a literal dash. It is at the start of the character class so that it is not interpreted as a class range. For example, [a-c] is a, b, c, but [-ac] is -, a, c

As an aside, this is an incomplete check for a filename. As far as I know, filenames can contain any character except directory separators (e.g. /) -- even spaces and newlines in any amount.

Upvotes: 3

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