正则表达式匹配字符串开头的确切子字符串

Question

I have one substring to be matched exactly at the beginning of source string.

source_string = "This is mat. This is cat."

substring1 = "This is"

substring2  = "That is"

source_string.match(/^(#{substring1}|#{substring2})$/)

This is what I tried it should work like this, if exact ' This is ' or ' That is ' is there at the beginning of string it should match, doesn't matter what is there after those substrings in source_string . My code is giving nil even if 'This is' is present.

Answer 1

I would not use a regex:

[substring1, substring2].any? { |sub| source_string.start_with?(sub) }
  #=> true

Answer 2

Remove $ at the end of the regular expression pattern.

source_string.match(/^(#{substring1}|#{substring2})$/)
                                                   ↑

By appending $ , it requires the pattern ends with This is or That is . You only need ^ at the beginning.

---

source_string = "This is mat. This is cat."
substring1 = "This is"
substring2  = "That is"
source_string.match(/^(#{substring1}|#{substring2})/)
# => #<MatchData "This is" 1:"This is">

Answer 3

While @falsetru is right about the core problem, the regexp is actually still wrong. Whilst the goal is to match a pattern at a beginning of source string , not at the beginning of each line , the \\A modifier should be used (see Regexp::Anchors for details):

source_string = <<-STR
Not to be matched. 
This is cat.
STR
source_string.match(/^This is/) # this should not be matched!
#⇒ #<MatchData "This is">
source_string.match(/\AThis is/)
#⇒ nil