The fact that you are searching for suggests you are either maintaining a historical system or working in a constrained environment. By following this guide, you can safely revive this classic lexer without compromising your system’s security or stability.
/usr/local/flex-2.5.4a/bin/flex --version download flex 2.5.4a
The web is littered with sketchy “download sites” that bundle malware. download precompiled binaries from random third-party mirrors. Instead, rely on trusted sources. The fact that you are searching for suggests
md5sum flex-2.5.4a.tar.gz # or sha1sum flex-2.5.4a.tar.gz download flex 2.5.4a
The GNU Flex project no longer actively hosts 2.5.4a on its primary website, but the files live on in historical archives.