($txt = '123abc') =~ /(a|b|c)/;

1. ^123abc  - check for a -> fail
            - check for b -> fail
            - check for c -> fail

2. 1^23abc  - check for a -> fail
            - check for b -> fail
            - check for c -> fail

3. 12^3abc  - check for a -> fail
            - check for b -> fail
            - check for c -> fail

4. 123^abc  - check for a -> succeess, $1 becomes 'a'

##</code><code>##

($txt = 'barefoot') =~ /(foo|foot)/;

1. ^barefoot  - check for foo  -> fail
              - check for foot -> fail

2. b^arefoot  - check for foo  -> fail
              - check for foot -> fail

3. ba^refoot  - check for foo  -> fail
              - check for foot -> fail

4. bar^efoot  - check for foo  -> fail
              - check for foot -> fail

5. bare^foot  - check for foo  -> success, $1 becomes 'foo'

##</code><code>##

($txt = 'arvec') =~ /(ar|ec|vec)$/

1. ^arvec  - check for ar$  -> fail (because of the end-of-string anchor)
           - check for ec$  -> fail
           - check for vec$ -> fail

2. a^rvec  - check for ar$  -> fail
           - check for ec$  -> fail
           - check for vec$ -> fail

3. ar^vec  - check for ar$  -> fail
           - check for ec$  -> fail
           - check for vec$ -> success, $1 becomes 'vec'