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call-seq:
scan(pattern) -> substring or nil
Attempts to [match][17] the given `pattern`
at the beginning of the [target substring][3].
If the match succeeds:
- Returns the matched substring.
- Increments the [byte position][2] by <tt>substring.bytesize</tt>,
and may increment the [character position][7].
- Sets [match values][9].
```rb
scanner = StringScanner.new(HIRAGANA_TEXT)
scanner.string # => "こんにちは"
scanner.pos = 6
scanner.scan(/に/) # => "に"
put_match_values(scanner)
# Basic match values:
# matched?: true
# matched_size: 3
# pre_match: "こん"
# matched : "に"
# post_match: "ちは"
# Captured match values:
# size: 1
# captures: []
# named_captures: {}
# values_at: ["に", nil]
# []:
# [0]: "に"
# [1]: nil
put_situation(scanner)
# Situation:
# pos: 9
# charpos: 3
# rest: "ちは"
# rest_size: 6
```
If the match fails:
- Returns `nil`.
- Does not increment byte and character positions.
- Clears match values.
```rb
scanner.scan(/nope/) # => nil
match_values_cleared?(scanner) # => true
```
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