What is the email matching regex in basic regex for grep?

Question

I created a text file and put some email addresses in it. Then I used grep to find them. Indeed it worked:

# pattern="^[a-zA-Z0-9]+@[a-zA-Z0-9]+\.[a-z]{2,}"
# grep -E $pattern regexfile

but only as long I kept the -E option for an extended regular expression. How do I need to change the above regex in order to use grep without -E option?

Solution

Be aware that matching email addresses is a LOT harder that what you have. See an excerpt from the Mastering Regular Expressions book

However, to answer your question, for a basic regular expression, your quantifiers need to be one of *, \+ or \{m,n\} (with the backslashes)

pattern='^[a-zA-Z0-9]\+@[a-zA-Z0-9]\+\.[a-z]\{2,\}'
grep "$pattern" regexfile

You need to quote the pattern variable

Comments

[Scott - Слава Україні]

BTW, in case glenn’s O’Reilly reference elicits a TL;DR response, this regex is way short of what you need to match real-world email addresses; see Email Address Syntax at Wikipedia, for starters.

[1574ad6]

Your regex isn't even close to correct for email address. For example, it doesn't recognize [email protected]. Many other examples.

[Abdul Al Hazred]

why is it important to quote the variable ?

[Angel Todorov]

The answer to that is spelled out in great detail here: unix.stackexchange.com/q/171346/4667