Even as a developer you may have to assemble a regular expression at runtime, at which point a library that can do it for you may be much more handy than having

Even as a developer you may have to assemble a regular expression at runtime, at which point a library that can do it for you may be much more handy than having to assemble the string yourself. And even if you know regex by heart - assembling it with function calls can still be better / safer just like you shouldn't insert SQL parameters by hand into your SQL query strings.