What is an XSO file? An XSO file is a regular expression project created by Ultrapico Expresso. It contains a regular expression, and it may contain a replacement string and sample text used to test the expression. XSO files can be opened, edited, and tested in Expresso.

More Information XSO file open in Ultrapico Expresso Expresso is a free .NET regular expression development app available for Windows. It allows you to design and test simple or complex regular expressions. As part of testing your expression, you can import and save sample text, used to determine whether your expression is behaving as desired. You can save your expression and the sample text together as an XSO project file, for future use in Expresso. After you've finished editing the expression your XSO file contains, you can export it from Expresso by selecting File → Save Regular Expression ... from the program's menu bar. This will save the expression you designed as a .TXT file. You can also import an existing expression into Expresso by selecting File → Read Regular Expression ... . (You can import and export sample text in a similar manner.)