Alternation and Grouping

Many a times, you want to check if the input string matches multiple patterns. For example, whether an object's color is green or blue or red. In programming terms, you need to perform OR conditional. This chapter will show how to use alternation for such cases. These patterns can have some common elements between them, in which case grouping helps to form terser expressions. This chapter will also discuss the precedence rules used to determine which alternation wins.

OR conditional

A conditional expression combined with logical OR evaluates to true if any of the condition is satisfied. Similarly, in regular expressions, you can use | metacharacter to combine multiple patterns to indicate logical OR. The matching will succeed if any of the alternate pattern is found in the input string. These alternatives have the full power of a regular expression, for example they can have their own independent anchors. Here's some examples.

# match either 'cat' or 'dog'
>> 'I like cats'.match?(/cat|dog/)
=> true
>> 'I like dogs'.match?(/cat|dog/)
=> true
>> 'I like parrots'.match?(/cat|dog/)
=> false

# replace either 'cat' at start of string or 'cat' at end of word
>> 'catapults concatenate cat scat'.gsub(/\Acat|cat\b/, 'X')
=> "Xapults concatenate X sX"

# replace either 'cat' or 'dog' or 'fox' with 'mammal'
>> 'cat dog bee parrot fox'.gsub(/cat|dog|fox/, 'mammal')
=> "mammal mammal bee parrot mammal"

Regexp.union method

You might infer from above examples that there can be cases where lots of alternation is required. The Regexp.union method can be used to build the alternation list automatically. It accepts an array as argument or a list of comma separated arguments.

>> Regexp.union('car', 'jeep')
=> /car|jeep/

>> words = %w[cat dog fox]
>> pat = Regexp.union(words)
>> pat
=> /cat|dog|fox/
>> 'cat dog bee parrot fox'.gsub(pat, 'mammal')
=> "mammal mammal bee parrot mammal"

In the above examples, the elements do not contain any special regular expression characters. Strings having metacharacters will be discussed in Regexp.escape method section.

Grouping

Often, there are some common things among the regexp alternatives. It could be common characters or regexp qualifiers like the anchors. In such cases, you can group them using a pair of parentheses metacharacters. Similar to a(b+c)d = abd+acd in maths, you get a(b|c)d = abd|acd in regular expressions.

# without grouping
>> 'red reform read arrest'.gsub(/reform|rest/, 'X')
=> "red X read arX"
# with grouping
>> 'red reform read arrest'.gsub(/re(form|st)/, 'X')
=> "red X read arX"

# without grouping
>> 'par spare part party'.gsub(/\bpar\b|\bpart\b/, 'X')
=> "X spare X party"
# taking out common anchors
>> 'par spare part party'.gsub(/\b(par|part)\b/, 'X')
=> "X spare X party"
# taking out common characters as well
# you'll later learn a better technique instead of using empty alternate
>> 'par spare part party'.gsub(/\bpar(|t)\b/, 'X')
=> "X spare X party"

There's plenty more features to grouping than just forming terser regexp. It will be discussed as they become relevant in coming chapters.

Regexp.source method

The Regexp.source method helps to interpolate a regexp literal inside another regexp. For example, adding anchors to alternation list created using the Regexp.union method.

>> words = %w[cat par]
>> alt = Regexp.union(words)
>> alt
=> /cat|par/
>> alt_w = /\b(#{alt.source})\b/
>> alt_w
=> /\b(cat|par)\b/

>> 'cater cat concatenate par spare'.gsub(alt, 'X')
=> "Xer X conXenate X sXe"
>> 'cater cat concatenate par spare'.gsub(alt_w, 'X')
=> "cater X concatenate X spare"

The above example will work without Regexp.source method too, but you'll see that /\b(#{alt})\b/ gives /\b((?-mix:cat|par))\b/ instead of /\b(cat|par)\b/. Their meaning will be explained in Modifiers chapter.

Precedence rules

There's some tricky situations when using alternation. If it is used for testing a match to get true/false against a string input, there is no ambiguity. However, for other things like string replacement, it depends on a few factors. Say, you want to replace either are or spared — which one should get precedence? The bigger word spared or the substring are inside it or based on something else?

In Ruby, the regexp alternative which matches earliest in the input string gets precedence. Regexp operator =~ helps to illustrate this concept.

>> words = 'lion elephant are rope not'

>> words =~ /on/
=> 2
>> words =~ /ant/
=> 10

# starting index of 'on' < index of 'ant' for given string input
# so 'on' will be replaced irrespective of order of regexp
>> words.sub(/on|ant/, 'X')
=> "liX elephant are rope not"
>> words.sub(/ant|on/, 'X')
=> "liX elephant are rope not"

So, what happens if two or more alternatives match on same index? The precedence is then left to right in the order of declaration.

>> mood = 'best years'

>> mood =~ /year/
=> 5
>> mood =~ /years/
=> 5

# starting index for 'year' and 'years' will always be same
# so, which one gets replaced depends on the order of alternation
>> mood.sub(/year|years/, 'X')
=> "best Xs"
>> mood.sub(/years|year/, 'X')
=> "best X"

Another example with gsub to drive home the issue.

>> words = 'ear xerox at mare part learn eye'

# this is going to be same as: gsub(/ar/, 'X')
>> words.gsub(/ar|are|art/, 'X')
=> "eX xerox at mXe pXt leXn eye"

# this is going to be same as: gsub(/are|ar/, 'X')
>> words.gsub(/are|ar|art/, 'X')
=> "eX xerox at mX pXt leXn eye"

# phew, finally this one works as needed
>> words.gsub(/are|art|ar/, 'X')
=> "eX xerox at mX pX leXn eye"

If you do not want substrings to sabotage your replacements, a robust workaround is to sort the alternations based on length, longest first.

>> words = %w[hand handy handful]

>> alt = Regexp.union(words.sort_by { |w| -w.length })
>> alt
=> /handful|handy|hand/

>> 'hands handful handed handy'.gsub(alt, 'X')
=> "Xs X Xed X"

# without sorting, order will come into play
>> 'hands handful handed handy'.gsub(Regexp.union(words), 'X')
=> "Xs Xful Xed Xy"

Cheatsheet and Summary

So, this chapter was about specifying one or more alternate matches within the same regexp using | metacharacter. Which can further be simplified using () grouping if there are common aspects among the alternations. Among the alternations, earliest matching pattern gets precedence. Left to right ordering is used as a tie-breaker if multiple alternations match starting from the same location. You also learnt couple of Regexp methods that help to programmatically construct a regexp literal.

Exercises

a) For the given input array, filter all elements that start with den or end with ly

>> items = ['lovely', "1\ndentist", '2 lonely', 'eden', "fly\n", 'dent']

>> items.grep()     ##### add your solution here
=> ["lovely", "2 lonely", "dent"]

b) For the given array, filter all elements having a line starting with den or ending with ly

>> items = ['lovely', "1\ndentist", '2 lonely', 'eden', "fly\nfar", 'dent']

>> items.grep()     ##### add your solution here
=> ["lovely", "1\ndentist", "2 lonely", "fly\nfar", "dent"]

c) For the given input strings, replace all occurrences of removed or reed or received or refused with X.

>> s1 = 'creed refuse removed read'
>> s2 = 'refused reed redo received'

>> pat =        ##### add your solution here

>> s1.gsub(pat, 'X')
=> "cX refuse X read"
>> s2.gsub(pat, 'X')
=> "X X redo X"

d) For the given input strings, replace all matches from the array words with A.

>> s1 = 'plate full of slate'
>> s2 = "slated for later, don't be late"
>> words = %w[late later slated]

>> pat =        ##### add your solution here

>> s1.gsub(pat, 'A')
=> "pA full of sA"
>> s2.gsub(pat, 'A')
=> "A for A, don't be A"

e) Filter all whole elements from the input array items that exactly matches any of the elements present in the array words.

>> items = ['slate', 'later', 'plate', 'late', 'slates', 'slated ']
>> words = %w[late later slated]

>> pat =        ##### add your solution here

>> items.grep(pat)
=> ["later", "late"]