Regsafe is a regular expression library which provides a type-safe and null-safe mechanism for capturing group extraction. The implementation makes extensive use of match types to statically analyze regular expressions during type checking. The Regsafe API mimics Scala's original regex APIs and is intended to be usable as a drop-in replacement.

The original implementation of this library is thoroughly documented in a Scala Symposium paper.

"in.nvilla" %% "regsafe" % "0.0.1"

There is a bug in the following code, can you spot it?

import regsafe.Regex

val date = Regex("""(\d{4})-(\d{2})-(\d{2})""")
"2004-01-20" match
  case date(y, m, d, a) =>
    s"$y was a good year for PLs."

With Regsafe, this is a compilation error:

[error] -- Error: src/test/scala/regsafe/neg.scala:9:14
[error] 9 |  case date(y, m, d, a) =>
[error]   |            ^
[error]   |this case is unreachable since type (String, String, String)
[error]   |is not a subclass of class Tuple4

The library derives the result type of regsafe.Regex.unapply by doing a type-level analysis of the regex literal singleton type. In this case, after counting parenthesis, the implementation concludes that "(\d{4})-(\d{2})-(\d{2})" has exactly 3 capturing groups and returns a (String, String, String), which is much more descriptive than the Seq[String] type used in the standard library's implementation.

Regsafe's type-level analysis also identifies nullable capturing groups. The unapply method nicely wraps those groups in options for null-safe consumption:

import regsafe.Regex

val rational = Regex("""(\d+)(?:\.(\d+))?""")
"3.1415" match
  case rational(i, Some(f)) =>
    val n = i.size + f.size
    s"This number is $n digits long"

Regsafe detect syntactically invalid regular expressions at compile time: (error messages are identical to those emitted by the JVM's runtime)

[error] 12 |  Regex("(")
[error]    |       ^
[error]    |       Unclosed group near index 1

[error] 13 |  Regex(")")
[error]    |       ^
[error]    |       Unmatched closing ')'

[error] 14 |  Regex("\\")
[error]    |       ^
[error]    |       Unexpected internal error near index 1