wildcard in regular expression in shell script

Question

I have a question regarding wildcard in shell script regular exression vi a.sh

if [[ $1 == 1*3 ]]; then
  echo "matching"
else
  echo "not matching"
fi

If I run sh a.sh 123 the output is: "matching".

But according to http://www.tldp.org/LDP/GNU-Linux-Tools-Summary/html/x11655.htm :

* (asterisk)

the proceeding item is to be matched zero or more times. ie. n* will match n, nn, nnnn, nnnnnnn but not na or any other character.

it should match to only 3,13,113,111..3.

But why is it matching 123?

Answer 1

In the documentation you linked you are taking the part in which it talks about "Regular expressions".

However, what is important here is what is within "Standard Wildcards (globbing patterns)":

Standard Wildcards (globbing patterns)

this can represent any number of characters (including zero, in other words, zero or more characters). If you specified a "cd*" it would use "cda", "cdrom", "cdrecord" and anything that starts with “cd” also including “cd” itself. "m*l" could by mill, mull, ml, and anything that starts with an m and ends with an l.

That is, it does not refer to the previous character but to a set of characters (zero or more). It is what equivalent to a .* in normal regular expressions.

So the expression 1*3 matches anything starting with 1 + zero or more characters + 3 at the end.

Answer 2

There are two different pattern matching you find in the Unix/Linux World:

• Regular Expressions : This is the complex pattern matching you find in grep , sed , and many other utilities. As time moved on, many extensions can be found. These extensions are referred to in POSIX as original (now considered obsolete), modern , and extended .
• Globbing : This refers to the file matching you find when you do things such as *.txt to match all text files. These are much simpler and less extensive. There are a few extensions (like ** to match subdirectories in Ant).

When you use [[ ... == ... ]] without quotes in Bash, you are using globbing file matches. If you want to use regular expressions, you need to use the =~ operator:

if [[ $foo =~ ^11*3 ]]   # Matches 13, 113, 1113, 11113
then