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check_until 

        # StringScanner.check_until

(from ruby core)
---
    check_until(pattern)

---

This returns the value that #scan_until would return, without advancing
the scan pointer.  The match register is affected, though.

    s = StringScanner.new("Fri Dec 12 1975 14:39")
    s.check_until /12/          # -> "Fri Dec 12"
    s.pos                       # -> 0
    s.matched                   # -> 12

Mnemonic: it "checks" to see whether a #scan_until will return a value.

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