fail2ban-regex: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/fail2ban-regex - from package: fail2ban

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/fail2ban-regex - from package: fail2ban

You may install the required package with command apt-get install {package}

Error in other languages:
fail2ban-regex: bevel nie gevind nie
fail2ban-regex: comandă negăsită
fail2ban-regex: comando não encontrado
fail2ban-regex: commande introuvable
fail2ban-regex: command not found
fail2ban-regex: käsku ei ole
fail2ban-regex: không tìm thấy lệnh
fail2ban-regex: komanda nerasta
fail2ban-regex: Kommando nicht gefunden.
fail2ban-regex: kommandot finns inte
fail2ban-regex: komut yok
fail2ban-regex: nie znaleziono polecenia
fail2ban-regex: níor aimsíodh an t-ordú
fail2ban-regex: no se encontró la orden
fail2ban-regex: no s'ha trobat l'ordre
fail2ban-regex: opdracht niet gevonden
fail2ban-regex: parancs nem található
fail2ban-regex: perintah tidak ditemukan
fail2ban-regex: príkaz nenájdený
fail2ban-regex: příkaz nenalezen
fail2ban-regex: команда не найдена
fail2ban-regex: командата не е открита
fail2ban-regex: 命令找不到
fail2ban-regex: コマンドが見つかりません