Regular Expression that matches a word, or nothing

Question

I'm really struggling to put a label on this, which is probably why I was unable to find what I need through a search.

I'm looking to match the following:

• Auto Reply
• Automatic Reply
• AutomaticReply

The platform that I'm using doesn't allow for the specification of case-insensitive searches. I tried the following regular expression:

.*[aA]uto(?:matic)[ ]*[rR]eply.*

Thinking that (?:matic) would cause my expression to match Auto or Automatic. However, it is only matching Automatic.

• What am I doing wrong?
• What is the proper terminology here?

This is using Perl for the regular expression engine (I think that's PCRE but I'm not sure).

Answer 1

(?:...) is to regex patterns as (...) is to arithmetic: It simply overrides precedence.

 ab|cd        # Matches ab or cd
 a(?:b|c)d    # Matches abd or acd

A ? quantifier is what makes matching optional.

 a?           # Matches a or an empty string
 abc?d        # Matches abcd or abd
 a(?:bc)?d    # Matches abcd or ad

You want

(?:matic)?

Without the needless leading and trailing .*, we get the following:

/[aA]uto(?:matic)?[ ]*[rR]eply/

---

As @adamdc78 points out, that matches AutoReply. This can be avoided as using the following:

/[aA]uto(?:matic[ ]*|[ ]+)[rR]eply/

or

/[aA]uto(?:matic|[ ])[ ]*[rR]eply/

Answer 2

This should work:

/.*[aA]uto(?:matic)? *[rR]eply/

you were simply missing the ? after (?:matic)