Regular Expression 3: Regex matching

This post covers basically the same things as Matching Regular Expressions but goes into a bit more detail. I recommend reading both posts since there is unique information in each.

The primary new item I show here is that more advanced matching techniques can be used but more importantly all groups are matched even groups that are within another group.

Note: The examples use Scala 2.8. Most examples will work with 2.7 but I believe the last example is Scala 2.8 only.

1. scala> val date = "11/01/2010"
2. date: java.lang.String = 11/01/2010
4. scala> val Date = """(\d\d)/(\d\d)/(\d\d\d\d)""".r
5. Date: scala.util.matching.Regex = (\d\d)/(\d\d)/(\d\d\d\d)
7. /*
8. When a Regex object is used in matching each group is assigned to a variable
9. */
10. scala> val Date(day, month, year) = date          
11. day: String = 11
12. month: String = 01
13. year: String = 2010
15. scala> val Date = """(\d\d)/((\d\d)/(\d\d\d\d))""".r
16. Date: scala.util.matching.Regex = (\d\d)/((\d\d)/(\d\d\d\d))
18. /*
19. This example demonstates how all groups must be assigned, if not there will be a matchError thrown
20. */
21. scala> val Date(day, monthYear, month, year) = date
22. day: String = 11
23. monthYear: String = 01/2010
24. month: String = 01
25. year: String = 2010
27. scala> val Date(day, month, year) = date           
28. scala.MatchError: 11/01/2010
29. at .< init>(< console>:5)
30. at .< clinit>(< console>)
33. // but placeholders work in Regex matching as well:
34. scala> val Date(day, _, month, year) = date
35. day: String = 11
36. month: String = 01
37. year: String = 2010
39. scala> val Names = """(\S+) (\S*)""".r             
40. Names: scala.util.matching.Regex = (\S+) (\S*)
42. scala> val Names(first, second) = "Jesse Eichar"
43. first: String = Jesse
44. second: String = Eichar
46. /*
47. If you want to use Regex's in assignment you must be sure the match will work.  Otherwise you should do real matching
48. */
49. scala> val Names(first, second) = "Jesse"       
50. scala.MatchError: Jesse
51. at .< init>(< console>:5)
52. at .< clinit>(< console>)
54. scala> val M = """\d{3}""".r
55. M: scala.util.matching.Regex = \d{3}
57. /*
58. There must be a group in the Regex or match will fail
59. */
60. scala> val M(m) = "Jan"
61. scala.MatchError: Jan
62. at .< init>(< console>:5)
63. at .< clinit>(< console>)

The following are a few more complex examples

1. scala> val Date = """((\d\d)/(\d\d)/(\d{4}))|((\w{3}) (\d\d),\s?(\d{4}))""".r         
2. Date: scala.util.matching.Regex = ((\d\d)/(\d\d)/(\d{4}))|((\w{3}) (\d\d),\s?(\d{4}))
4. /*
5. The Regex has an or in it.  So only 1/2 of the groups will be non-null.
6. If the first group is a String then it is non-null and the next three elements
7. the pattern will be day/month/year
9. Otherwise if the 5th group is a String then the patter will be month day, year
11. Lastly a catch all
12. */
13. scala> def printDate(date:String) = date match {                                      
14.      | case Date(_:String,day,month,year,_,_,_,_) => (day,month,year)                 
15.      | case Date(_,_,_,_,_:String,month,day,year) => (day,month,year) // process month
16.      | case _ => ("x","x","x")                                                        
17.      | }
18. printDate: (date: String)(String, String, String)
20. scala> printDate("Jan 01,2010") 
21. res0: (String, String, String) = (01,Jan,2010)
23. scala> printDate("01/01/2010") 
24. res1: (String, String, String) = (01,01,2010)
26. /*
27. A silly example which drops the first element of the date string
28. not useful but this demonstrates that we are matching agains a sequence so 
29. the _* can be used to match the rest of the groups
30. */
31. scala> def split(date:String) = date match {         
32.      | case d @ Date(_:String ,_*) => d drop 3       
33.      | case d @ Date(_,_,_,_,_:String,_*) => d drop 4
34.      | case _ => "boom"                              
35.      | }
36. split: (date: String)String
38. scala> split ("Jan 31,2004")
39. res5: String = 31,2004
41. scala> split ("11/12/2004") 
42. res6: String = 12/2004
44. /*
45. This is just a reminder that the findAllIn returns an iterator which (since it is probably a short iterator) can be converted to a sequence and processed with matching
46. */
47. scala> val Seq(one,two,_*) = ("""\d\d/""".r findAllIn "11/01/2010" ).toSeq  
48. one: String = 11/
49. two: String = 01/
51. scala> val Seq(one,two) = ("""\d\d/""".r findAllIn "11/01/2010" ).toSeq   
52. one: String = 11/
53. two: String = 01/
55. // drop the two first matches and assign the rest to d
56. scala> val Seq(_,_,d @ _*) = ("""\d\d/""".r findAllIn "11/01/20/10/" ).toSeq
57. d: Seq[String] = ArrayBuffer(20/, 10/)