Trigger function regex() incorrectly matches regular expressions

I have a trapper item of type Text and a simple trigger attached to it. Here is my trigger configuration:

According to my understanding this regular expression should match any string containing "a" or "b" or "c". But for some reason it does not fire when I send "a", "b" or "c". It only fires when I send "abc":

In emails I get:

Trigger: Last value matches regexp
Trigger status: PROBLEM
Trigger severity: Not classified
Trigger URL:

Item values:

1. Text trapper (test:text.trapper): abc
2. UNKNOWN (UNKNOWN:UNKNOWN): UNKNOWN
3. UNKNOWN (UNKNOWN:UNKNOWN): UNKNOWN

Original event ID: 164

and

Trigger: Last value matches regexp
Trigger status: OK
Trigger severity: Not classified
Trigger URL:

Item values:

1. Text trapper (test:text.trapper): a
2. UNKNOWN (UNKNOWN:UNKNOWN): UNKNOWN
3. UNKNOWN (UNKNOWN:UNKNOWN): UNKNOWN

Original event ID: 164