Class: StringScanner

Relationships & Source Files
Namespace Children
Exceptions: Error
Inherits:	Object
Defined in:	ext/strscan/strscan.c, ext/strscan/strscan.c

Overview

StringScanner provides for lexical scanning operations on a String. Here is an example of its usage:

s = StringScanner.new('This is an example string')
s.eos?               # -> false

p s.scan(/\w+/)      # -> "This"
p s.scan(/\w+/)      # -> nil
p s.scan(/\s+/)      # -> " "
p s.scan(/\s+/)      # -> nil
p s.scan(/\w+/)      # -> "is"
s.eos?               # -> false

p s.scan(/\s+/)      # -> " "
p s.scan(/\w+/)      # -> "an"
p s.scan(/\s+/)      # -> " "
p s.scan(/\w+/)      # -> "example"
p s.scan(/\s+/)      # -> " "
p s.scan(/\w+/)      # -> "string"
s.eos?               # -> true

p s.scan(/\s+/)      # -> nil
p s.scan(/\w+/)      # -> nil

Scanning a string means remembering the position of a scan pointer, which is just an index. The point of scanning is to move forward a bit at a time, so matches are sought after the scan pointer; usually immediately after it.

Given the string “test string”, here are the pertinent scan pointer positions:

t e s t   s t r i n g
0 1 2 ...             1
                      0

When you #scan for a pattern (a regular expression), the match must occur at the character after the scan pointer. If you use #scan_until, then the match can occur anywhere after the scan pointer. In both cases, the scan pointer moves just beyond the last character of the match, ready to scan again from the next character onwards. This is demonstrated by the example above.

Method Categories

There are other methods besides the plain scanners. You can look ahead in the string without actually scanning. You can access the most recent match. You can modify the string being scanned, reset or terminate the scanner, find out or change the position of the scan pointer, skip ahead, and so on.

There are aliases to several of the methods.

Class Method Summary

.must_C_version

This method is defined for backward compatibility.
.new(string, dup = false) constructor private

Creates a new StringScanner object to scan over the given #string.

Instance Attribute Summary

#pointer rw

Alias for #pos.
#pos (also: #pointer) rw

Returns the byte position of the scan pointer.
#pos=(n) (also: #pointer=) rw

Set the byte position of the scan pointer.
#string rw

Returns the string being scanned.
#string=(str) rw

Changes the string being scanned to str and resets the scanner.
#beginning_of_line? ⇒ Boolean readonly

Returns true iff the scan pointer is at the beginning of the line.
#empty? ⇒ Boolean readonly

Equivalent to #eos?.
#eos? ⇒ Boolean readonly

Returns true if the scan pointer is at the end of the string.
#matched readonly

Returns the last matched string.
#matched? ⇒ Boolean readonly

Returns true iff the last match was successful.
#rest readonly

Returns the “rest” of the string (i.e. everything after the scan pointer).
#rest? ⇒ Boolean readonly

Returns true iff there is more data in the string.

Instance Method Summary

#<<(str) (also: #concat)

Appends str to the string being scanned.
#[](n)

Return the n-th subgroup in the most recent match.
#charpos

Returns the character position of the scan pointer.
#check(pattern)

This returns the value that #scan would return, without advancing the scan pointer.
#check_until(pattern)

This returns the value that #scan_until would return, without advancing the scan pointer.
#clear

Equivalent to #terminate.
#concat(str)

Alias for #<<.
#exist?(pattern)

Looks ahead to see if the pattern exists anywhere in the string, without advancing the scan pointer.
#get_byte

Scans one byte and returns it.
#getbyte

Equivalent to #get_byte.
#getch

Scans one character and returns it.
#inspect

Returns a string that represents the StringScanner object, showing: - the current position - the size of the string - the characters surrounding the scan pointer.
#match?(pattern)

Tests whether the given pattern is matched from the current scan pointer.
#matched_size

Returns the size of the most recent match (see #matched), or nil if there was no recent match.
#peek(len)

Extracts a string corresponding to string[pos,len], without advancing the scan pointer.
#peep(vlen)

Equivalent to #peek.
#post_match

Return the post-match (in the regular expression sense) of the last scan.
#pre_match

Return the pre-match (in the regular expression sense) of the last scan.
#reset

Reset the scan pointer (index 0) and clear matching data.
#rest_size

s.rest_size is equivalent to s.rest.size.
#restsize

s.restsize is equivalent to s.rest_size.
#scan(pattern) ⇒ String

Tries to match with pattern at the current position.
#scan_full(pattern, advance_pointer_p, return_string_p)

Tests whether the given pattern is matched from the current scan pointer.
#scan_until(pattern)

Scans the string until the pattern is matched.
#search_full(pattern, advance_pointer_p, return_string_p)

Scans the string until the pattern is matched.
#skip(pattern)

Attempts to skip over the given pattern beginning with the scan pointer.
#skip_until(pattern)

Advances the scan pointer until pattern is matched and consumed.
#terminate

Set the scan pointer to the end of the string and clear matching data.
#unscan

Set the scan pointer to the previous position.
#dup private

Duplicates a StringScanner object.

Constructor Details

.new(string, dup = false) (private)

Creates a new StringScanner object to scan over the given #string. dup argument is obsolete and not used now.

Class Method Details

.must_C_version

This method is defined for backward compatibility.

Instance Attribute Details

#beginning_of_line? ⇒ `Boolean` (readonly)

Returns true iff the scan pointer is at the beginning of the line.

s = StringScanner.new("test\ntest\n")
s.bol?           # => true
s.scan(/te/)
s.bol?           # => false
s.scan(/st\n/)
s.bol?           # => true
s.terminate
s.bol?           # => true

#empty? ⇒ `Boolean` (readonly)

Equivalent to #eos?. This method is obsolete, use #eos? instead.

#eos? ⇒ `Boolean` (readonly)

Returns true if the scan pointer is at the end of the string.

s = StringScanner.new('test string')
p s.eos?          # => false
s.scan(/test/)
p s.eos?          # => false
s.terminate
p s.eos?          # => true

#matched (readonly)

Returns the last matched string.

s = StringScanner.new('test string')
s.match?(/\w+/)     # -> 4
s.matched           # -> "test"

#matched? ⇒ `Boolean` (readonly)

Returns true iff the last match was successful.

s = StringScanner.new('test string')
s.match?(/\w+/)     # => 4
s.matched?          # => true
s.match?(/\d+/)     # => nil
s.matched?          # => false

#pointer (rw)

Alias for #pos.

#pos (rw) Also known as: #pointer

Returns the byte position of the scan pointer. In the 'reset' position, this value is zero. In the 'terminated' position (i.e. the string is exhausted), this value is the bytesize of the string.

In short, it's a 0-based index into bytes of the string.

s = StringScanner.new('test string')
s.pos               # -> 0
s.scan_until /str/  # -> "test str"
s.pos               # -> 8
s.terminate         # -> #<StringScanner fin>
s.pos               # -> 11

#pos=(n) (rw) Also known as: #pointer=

Set the byte position of the scan pointer.

s = StringScanner.new('test string')
s.pos = 7            # -> 7
s.rest               # -> "ring"

#rest (readonly)

Returns the “rest” of the string (i.e. everything after the scan pointer). If there is no more data (eos? = true), it returns "".

#rest? ⇒ `Boolean` (readonly)

Returns true iff there is more data in the string. See #eos?. This method is obsolete; use #eos? instead.

s = StringScanner.new('test string')
s.eos?              # These two
s.rest?             # are opposites.

#string (rw)

Returns the string being scanned.

#string=(str) (rw)

Changes the string being scanned to str and resets the scanner. Returns str.

Instance Method Details

#concat(str) #<<(str)
Also known as: #concat

Appends str to the string being scanned. This method does not affect scan pointer.

s = StringScanner.new("Fri Dec 12 1975 14:39")
s.scan(/Fri /)
s << " +1000 GMT"
s.string            # -> "Fri Dec 12 1975 14:39 +1000 GMT"
s.scan(/Dec/)       # -> "Dec"

#[](n)

Return the n-th subgroup in the most recent match.

s = StringScanner.new("Fri Dec 12 1975 14:39")
s.scan(/(\w+) (\w+) (\d+) /)       # -> "Fri Dec 12 "
s[0]                               # -> "Fri Dec 12 "
s[1]                               # -> "Fri"
s[2]                               # -> "Dec"
s[3]                               # -> "12"
s.post_match                       # -> "1975 14:39"
s.pre_match                        # -> ""

s.reset
s.scan(/(?<wday>\w+) (?<month>\w+) (?<day>\d+) /)       # -> "Fri Dec 12 "
s[0]                               # -> "Fri Dec 12 "
s[1]                               # -> "Fri"
s[2]                               # -> "Dec"
s[3]                               # -> "12"
s[:wday]                           # -> "Fri"
s[:month]                          # -> "Dec"
s[:day]                            # -> "12"
s.post_match                       # -> "1975 14:39"
s.pre_match                        # -> ""

#charpos

Returns the character position of the scan pointer. In the 'reset' position, this value is zero. In the 'terminated' position (i.e. the string is exhausted), this value is the size of the string.

In short, it's a 0-based index into the string.

s = StringScanner.new("abcädeföghi")
s.charpos           # -> 0
s.scan_until(/ä/)   # -> "abcä"
s.pos               # -> 5
s.charpos           # -> 4

#check(pattern)

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

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

Mnemonic: it “checks” to see whether a #scan will return a value.

#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.

#clear

Equivalent to #terminate. This method is obsolete; use #terminate instead.

#concat(str) #<<(str)

Alias for #<<.

#exist?(pattern)

Looks ahead to see if the pattern exists anywhere in the string, without advancing the scan pointer. This predicates whether a #scan_until will return a value.

s = StringScanner.new('test string')
s.exist? /s/            # -> 3
s.scan /test/           # -> "test"
s.exist? /s/            # -> 2
s.exist? /e/            # -> nil

#get_byte

Scans one byte and returns it. This method is not multibyte character sensitive. See also: #getch.

s = StringScanner.new('ab')
s.get_byte         # => "a"
s.get_byte         # => "b"
s.get_byte         # => nil

$KCODE = 'EUC'
s = StringScanner.new("\244\242")
s.get_byte         # => "\244"
s.get_byte         # => "\242"
s.get_byte         # => nil

#getbyte

Equivalent to #get_byte. This method is obsolete; use #get_byte instead.

#getch

Scans one character and returns it. This method is multibyte character sensitive.

s = StringScanner.new("ab")
s.getch           # => "a"
s.getch           # => "b"
s.getch           # => nil

$KCODE = 'EUC'
s = StringScanner.new("\244\242")
s.getch           # => "\244\242"   # Japanese hira-kana "A" in EUC-JP
s.getch           # => nil

#dup (private) #clone

Duplicates a StringScanner object.

#inspect

Returns a string that represents the StringScanner object, showing:

the current position
the size of the string
the characters surrounding the scan pointer

s = StringScanner.new(“Fri Dec 12 1975 14:39”) s.inspect # -> '#<StringScanner 0/21 @ “Fri D…”>' s.scan_until /12/ # -> “Fri Dec 12” s.inspect # -> '#<StringScanner 10/21 “…ec 12” @ “ 1975…”>'

#match?(pattern)

Tests whether the given pattern is matched from the current scan pointer. Returns the length of the match, or nil. The scan pointer is not advanced.

s = StringScanner.new('test string')
p s.match?(/\w+/)   # -> 4
p s.match?(/\w+/)   # -> 4
p s.match?(/\s+/)   # -> nil

#matched_size

Returns the size of the most recent match (see #matched), or nil if there was no recent match.

s = StringScanner.new('test string')
s.check /\w+/           # -> "test"
s.matched_size          # -> 4
s.check /\d+/           # -> nil
s.matched_size          # -> nil

#peek(len)

Extracts a string corresponding to string[pos,len], without advancing the scan pointer.

s = StringScanner.new('test string')
s.peek(7)          # => "test st"
s.peek(7)          # => "test st"

#peep(vlen)

Equivalent to #peek. This method is obsolete; use #peek instead.

#post_match

Return the post-match (in the regular expression sense) of the last scan.

s = StringScanner.new('test string')
s.scan(/\w+/)           # -> "test"
s.scan(/\s+/)           # -> " "
s.pre_match             # -> "test"
s.post_match            # -> "string"

#pre_match

Return the pre-match (in the regular expression sense) of the last scan.

s = StringScanner.new('test string')
s.scan(/\w+/)           # -> "test"
s.scan(/\s+/)           # -> " "
s.pre_match             # -> "test"
s.post_match            # -> "string"

#reset

Reset the scan pointer (index 0) and clear matching data.

#rest_size

s.rest_size is equivalent to s.rest.size.

#restsize

s.restsize is equivalent to s.rest_size. This method is obsolete; use #rest_size instead.

#scan(pattern) ⇒ `String`

Tries to match with pattern at the current position. If there's a match, the scanner advances the “scan pointer” and returns the matched string. Otherwise, the scanner returns nil.

s = StringScanner.new('test string')
p s.scan(/\w+/)   # -> "test"
p s.scan(/\w+/)   # -> nil
p s.scan(/\s+/)   # -> " "
p s.scan(/\w+/)   # -> "string"
p s.scan(/./)     # -> nil

#scan_full(pattern, advance_pointer_p, return_string_p)

Tests whether the given pattern is matched from the current scan pointer. Advances the scan pointer if advance_pointer_p is true. Returns the matched string if return_string_p is true. The match register is affected.

“full” means “#scan with full parameters”.

#scan_until(pattern)

Scans the string until the pattern is matched. Returns the substring up to and including the end of the match, advancing the scan pointer to that location. If there is no match, nil is returned.

s = StringScanner.new("Fri Dec 12 1975 14:39")
s.scan_until(/1/)        # -> "Fri Dec 1"
s.pre_match              # -> "Fri Dec "
s.scan_until(/XYZ/)      # -> nil

#search_full(pattern, advance_pointer_p, return_string_p)

Scans the string until the pattern is matched. Advances the scan pointer if advance_pointer_p, otherwise not. Returns the matched string if return_string_p is true, otherwise returns the number of bytes advanced. This method does affect the match register.

#skip(pattern)

Attempts to skip over the given pattern beginning with the scan pointer. If it matches, the scan pointer is advanced to the end of the match, and the length of the match is returned. Otherwise, nil is returned.

It's similar to #scan, but without returning the matched string.

s = StringScanner.new('test string')
p s.skip(/\w+/)   # -> 4
p s.skip(/\w+/)   # -> nil
p s.skip(/\s+/)   # -> 1
p s.skip(/\w+/)   # -> 6
p s.skip(/./)     # -> nil

#skip_until(pattern)

Advances the scan pointer until pattern is matched and consumed. Returns the number of bytes advanced, or nil if no match was found.

Look ahead to match pattern, and advance the scan pointer to the end of the match. Return the number of characters advanced, or nil if the match was unsuccessful.

It's similar to #scan_until, but without returning the intervening string.

s = StringScanner.new("Fri Dec 12 1975 14:39")
s.skip_until /12/           # -> 10
s                           #

#terminate #clear

Set the scan pointer to the end of the string and clear matching data.

#unscan

Set the scan pointer to the previous position. Only one previous position is remembered, and it changes with each scanning operation.

s = StringScanner.new('test string')
s.scan(/\w+/)        # => "test"
s.unscan
s.scan(/../)         # => "te"
s.scan(/\d/)         # => nil
s.unscan             # ScanError: unscan failed: previous match record not exist