How to get parts of a string via regular expressions in Kotlin or Java?

Question

Suppose you have a string http://server.com/s123456 and want to get 123456 from it. Or http://server.com/s123456&a=100 and get 123456, 100. Many different situations where you want to have parts of a string.

In order to match strings I use regular expressions.

@Test
fun test() {
    val text = "http://server.com/s123456" // Source string.
    val reg = "^http://server.com/s\\d+\$".toRegex()
    assertEquals(true, reg.matches(text))

    // Try to split the string with findAll().
    val a: Sequence<MatchResult> = reg.findAll(text, 0)
    val names = a.map { it.groupValues[0] }.joinToString()
    println(names) // http://server.com/s123456
}

But it doesn't split the source string into chars and digits, just prints: http://server.com/s123456. Is there a way to write a mask, so that we can retrieve numbers (or any other parts) from the string?

Im not familiar with kotlin, but You seems to not having groups. "^http://server.com/s(\\d+)\$" try this and use 1 in groupValues index. — Eraklon
– Eraklon, Commented Feb 17, 2020 at 14:44
@Eraklon, thanks, it works! Please, create an answer here, so that I could accept it. — CoolMind
– CoolMind, Commented Feb 17, 2020 at 14:57

Eraklon · Accepted Answer · 2020-02-17 14:59:18Z

1

You need to use capture group around the the part that you interested. Use this "^http://server.com/s(\\d+)\$" and use it.groupValues[1].

answered Feb 17, 2020 at 14:59

Eraklon

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moys · Accepted Answer · 2020-02-17 14:57:29Z

1

You can try just (\d+)

Example (written in Python):

a = 'http://server.com/s123456'
b = 'http://server.com/s123456&a=100'
c = 'http://server.com/s122&&&'
d = 'http://server.com/s1234!56&a=100terere'
lis=[a,b,c,d]
for item in lis:
    print(re.findall(r'(\d+)', item ))

output

['123456']
['123456', '100']
['122']
['1234', '56', '100']

answered Feb 17, 2020 at 14:57

moys

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CoolMind · Accepted Answer · 2020-02-17 15:57:29Z

Thanks to @Eraklon and @moys I wrote a similar answer.

@Test
fun test1() {
    val text = "http://server.com/s123456"
    val reg = "^http://server.com/s(\\d+)\$".toRegex()
    val a = reg.findAll(text, 0)
    val names = a.map { it.groupValues[1] }.joinToString()
    println(names) 
    // 123456
    assertEquals(true, reg.matches(text))
}

@Test
fun test2() {
    val a = "http://server.com/s123456"
    val b = "http://server.com/s123456&a=100"
    val c = "http://server.com/s122&&&"
    val d = "http://server.com/s1234!56&a=100terere"
    val e = "http://server.com/s"
    val f = "http://server.com"
    val reg = "(\\d+)".toRegex()

    val list = listOf(a, b, c, d, e, f)
    list.map { item ->
        val groups: Sequence<MatchResult> = reg.findAll(item, 0)
        val names = groups.map { it.groupValues[1] }.joinToString()
        println(names)
    }
    // Output:
    // 123456
    // 123456, 100
    // 122
    // 1234, 56, 100
    // 
    // 

    val groups: Sequence<MatchResult> = reg.findAll(e, 0)
    val names = groups.map { it.groupValues[1] }.firstOrNull()
    println(names)
    // null
}

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