Ruby regex match starting at specific position

Question

In Python I can do this:

import re
regex = re.compile('a')

regex.match('xay',1)  # match because string starts with 'a' at 1
regex.match('xhay',1) # no match because character at 1 is 'h'

However in Ruby, the match method seems to match everything past the positional argument. For instance, /a/.match('xhay',1) will return a match, even though the match actually starts at 2. However, I want to only consider matches that start at a specific position.

How do I get a similar mechanism in Ruby? I would like to match patterns that start at a specific position in the string as I can in Python.

Answer 1

/^.{1}a/

for matching a at location x+1 in the string

/^.{x}a/

--> DEMO

Answer 2

How about below using StringScanner ?

require 'strscan'

scanner =  StringScanner.new 'xay'
scanner.pos = 1
!!scanner.scan(/a/) # => true

scanner =  StringScanner.new 'xnnay'
scanner.pos = 1
!!scanner.scan(/a/) # => false

Answer 3

Regexp#match has an optional second parameter pos , but it works like Python's search method. You could however check if the returned MatchData begins at the specified position:

re = /a/

match_data = re.match('xay', 1)
match_data.begin(0) == 1
#=> true

match_data = re.match('xhay', 1)
match_data.begin(0) == 1
#=> false

match_data = re.match('áay', 1)
match_data.begin(0) == 1
#=> true

match_data = re.match('aay', 1)
match_data.begin(0) == 1
#=> true

Answer 4

Extending a little bit on what @sunbabaphu answered:

def matching_at_pos(x=0, regex)
  /\A.{#{x-1}}#{regex}/ 
end # note the position is 1 indexed

'xxa' =~ matching_at_pos(2, /a/)
=> nil
'xxa' =~ matching_at_pos(3, /a/)
=> 0
'xxa' =~ matching_at_pos(4, /a/)
=> nil

Answer 5

The answer to this question is \G .

\G matches the starting point of the regex match, when calling the two-argument version of String#match that takes a starting position.

'xay'.match(/\Ga/, 1) # match because /a/ starts at 1
'xhay'match(/\Ga/, 1) # no match because character at 1 is 'h'