最初の下線で始まり、最初のハイフンで終わるファイル名を抽出します。

man grep説明する

  grep searches for PATTERNS in each FILE.  PATTERNS is one or patterns separated by newline characters, and grep prints each line that matches a pattern.

 -o, --only-matching
              Print  only  the  matched  (non-empty) parts of a matching line,
              with each such part on a separate output line.

 -P, --perl-regexp
              Interpret   PATTERNS   as  Perl-compatible  regular  expressions
              (PCREs).  This option is experimental when combined with the  -z
              (--null-data)  option,  and  grep  -P  may warn of unimplemented features.

私がls持っている

'2180_PP AAA Radius Statistic-42005_04May2020_0900-04May2020_1000.csv'
'2180_SW Interface  Flow(3GPP AAA)-53448_14May2020_0000-14May2020_0100.csv'

以下のコードを実行すると

ls | grep -oP '(?<=_).*(?=\-\d\d\d)'

PP AAA Radius Statistic
SW Interface  Flow(3GPP AAA)

説明REGEX

(?<= - Stands for a positive look-behind and will not include the words before it

.    - Matches any characters except line break

(?=  - Stands for positive look-ahead. Matches a group 
       after the main result without including it in the result.

\-   - Matched character -

\d   - Matched digit

REGEX 説明のソースは次のとおりです。正規表現

なぜ他の結果が出るのですか？

-入力（-14May）に別の一致がありますか？だから私は\-\d\d\dそれに抵抗した。