Hackerrank Regular Expression Challenges
Solutions to Hackerrank's Regular Expression Challenges at https://www.hackerrank.com/domains/regex/
Last revision: 22 October 2017
- Introduction
- Character Class
- Repetitions Pre-production
- Grouping and Capturing Pre-production
- Backreferences Pre-production
- Assertions Pre-production
- Applications Pre-production
Introduction
- Matching Specific String
- Matching Anything But a Newline
- Matching Digits & Non-Digit Characters
- Matching Whitespace & Non-Whitespace Character
- Matching Word & Non-Word Character
- Matching Start & End
Matching Specific String
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Task
You have a test string S. Your task is to match the string hackerrank. This is case sensitive.
Solution
# Use a regex made up of the given string.
# In Python, if your string is treated as a Regex if it has an "r" before the first quote.
Regex_Pattern = r"hackerrank" Matching Anything But a Newline
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Task
You have a test string S.
Your task is to match the pattern abc.def.ghi.jkx, and only exact strings that follow this pattern
where each variable a,b,c,d,e,f,g,h,i,j,k,x can be any single character except the newline.
Solution
# Because we want to match exact patterns in the string, we'll have
# The caret, ^, matches the start of the line.
# And similarly, the dollar sign, $, matches the end of the line.
# A dot matches anything except for a newline character.
# And to match a dot we need to escape it using a backslash, like this: \.
# So the following regex matches:
# A line that has 3 non-newline characters, followed by a dot, 3 non-newline characters again,
# then a dot, 3 non-newline characters again a dot and then 3 non-newline characters and nothing else.
regex_pattern = r"^...\....\....\....$"Matching Digits & Non-Digit Characters
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Task
You have a test string S. Your task is to match the pattern xxXxxXxxxx
Here x denotes a digit character, and X denotes a non-digit character.
Solution
# \d matches a digit character [0,1,2,3,4,5,6,7,8,9]
# \D matches a non-digit character.
# So the following regex matches a sequence of
# 2 digits, 1 non-digit, 2 digit, 1 non-digit and 4 digits.
Regex_Pattern = r"\d\d\D\d\d\D\d\d\d\d"Matching Whitespace & Non-Whitespace Character
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Task
You have a test string S. Your task is to match the pattern XXxXXxXX
Here, x denotes whitespace characters, and X denotes non-white space characters.
Solution
# \s matches a whitespace character, one of [ \r \n \t \f ]
# \S matches a non-whitespace character.
# So the following regex matches a sequence of
# 2 Non-white space characters, a whitespace character,
# 2 Non-white space characters, a whitespace character,
# 2 Non-white space characters.
Regex_Pattern = r"\S\S\s\S\S\s\S\S"Matching Word & Non-Word Character
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Task
You have a test string S. Your task is to match the pattern xxxXxxxxxxxxxxXxxx
Here x denotes any word character and X denotes any non-word character.
Solution
# \w matches a word character, it could be a letter, a digit or an underscore.
# And \w matches a non-word character, anything that other than what \w matches.
# It's time to use a trick though now,
# While for instance \w matches 1 word character,
# \w{2} matches 2 word characters, and \w{n} matches
# n word characters.
# So the following pattern matches
# 3 word characters, 1 non-words, 10 word characters,
# 1 non-word character and finally 3 word characters.
Regex_Pattern = r"\w{3}\W\w{10}\W\w{3}"
# Alternatively
# Regex_Pattern = r"\w\w\w\W\w\w\w\w\w\w\w\w\w\w\W\w\w\w"Matching Start & End
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Task
You have a test string S. Your task is to match the pattern Xxxxx.
Here, x denotes a word character, and X denotes a digit.
S must start with a digit X and end with . symbol.
S should be 6 characters long only.
Solution
# ^ matches the line start.
# There is 1 digit following the line start.
# Which is followed by 4 word characters.
# Then there is a period, which is escaped with a backslash.
# Then the regex matches the line end.
Regex_Pattern = r"^\d\w{4}\.$"Character Class
Matching Specific Characters
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Task
Your test string S will have the following requirements:
- S must be of length 6
- First character: 1, 2 or 3
- Second character: 1, 2 or 0
- Third character: x, s or 0
- Fourth character: 3, 0, A or a
- Fifth character: x, s or u
- Sixth character: . or ,
Solution
# ^ matches the line start.
# Any character from inside the square brackets can be matched with one character from the string.
# [123] can match 1, 2 or 3 in the string.
# Each group of characters inside the square brackets matches with one character in the string,
# There are 6 groups of characters surrounded by square brackets, each group to match one character from the string,
# Thus ensuring the length of the string is 6, since we have the line start and line end at the ends of our pattern.
Regex_Pattern = r'^[123][120][xs0][30Aa][xsu][\.,]$'Excluding Specific Characters
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Task
Your test string S will have the following requirements:
- S must be of length 6
- First character should not be a digit (1, 2, 3, 4, 5, 6, 7, 8, 9 or 0).
- Second character should not be a lowercase vowel (a, e, i, o or u).
- Third character should not be b, c, D or E.
- Fourth character should not be a whitespace character (\r, \n, \t, \f or the space character).
- Fifth character should not be an uppercase vowel (A, E, I, O or U).
- Sixth character should not be a . or , symbol.
Solution
# ^ matches the line start.
# A caret, ^, inside the square brackets match a character from the string as long as that character is not found in the square bracket from the pattern.
# [^aeiou] will match any character that is not a, e, i, o or u.
# [^\d] will match any non-digit character, like \D.
# Again there are 6 groups of characters surrounded by square brackets,
# Thus ensuring the length of the string is 6, since we have the line start and line end at the ends of our pattern.
Regex_Pattern = r'^[^\d][^aeiou][^bcDF][\S][^AEIOU][^\.\,]$'Matching Character Ranges
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Task
- The test string's length is greater than or equal to 5.
- The first character must be a lowercase English letter.
- The second character is a positive digit, cannot be zero.
- The third character must not be a lowercase English letter.
- The fourth character must not be an uppercase English letter.
- The fifth character must be an uppercase English letter.
Solution
# ^ matches the line start. We need the line start because we're matching starting from the first character of the string.
# [a-z] will match any lowercase English letter from a to z both inclusive. [b-z] and [a-y] would match all lowercase English letters except for a and z respectively.
# [1-9] will match any digit except for zero.
# [^a-z] will match any character that is not a lowercase English letter.
# [^A-Z] will match any character that is not an uppercase English letter.
# [A-Z] will match any character that is an uppercase English letter.
# Note that for this pattern there is no line end, because only the first five characters of the string are relevant, the rest may be anything.
Regex_Pattern = r'^[a-z][1-9][^a-z][^A-Z][A-Z]'Repetitions
Grouping and Capturing
Backreferences
Assertions
Applications
Revisions
- 1 January 2017 - Added Introduction
- 22 October 2017 - Added Character Class