Ruby Sub, gsub: Replace String

Sub, gsub. String replacements can be done with the sub() or gsub methods.

These are substitution methods. Gsub applies the substitution globally.

Regexp, strings. We can use strings or regular expressions as the arguments to these methods. With special codes like "\1" we can insert parts of the match into a replacement.

Simple example. Let us begin with this example. We use "sub!" which replaces inline, and sub() which returns another string. In these calls, the first argument is replaced with the second.

Result: The string "value" has its matching characters replaced according to sub's arguments.

Ruby program that uses sub method

value = "cat"

# Replace string at with ote.
value.sub!("at", "ote")
puts value

value = "bird"

# This version of sub does not change the string in-place.
value = value.sub("ird", "ark")
puts value

Output

cote
bark

Sub versus gsub. The sub() method replaces just the first instance of a string with another. Gsub meanwhile replaces all instances.

Thus: Gsub is closest to a "replace string" method. Sub() is conceptually a "replace first string" method.

Ruby program that compares sub, gsub

value = "abc abc"
puts value

# Sub replaces just the first instance.
value = value.sub("abc", "---")
puts value

# Gsub replaces all instances.
value = value.gsub("abc", "---")
puts value

Output

abc abc
--- abc
--- ---

Substring. We can assign a substring within a string. This syntax has the same effect as calling sub() on a string. It only replaces the first match.

Tip: Regular expressions, ranges, and indexes can be used. Substring assignment is versatile and easy to do.

Substring: assign

Ruby program that uses substring assignment

value = "abc abc"
puts value

# A substring can be changed within a string.
# ... Only the first instance is replaced.
value["abc"] = "def"
puts value

Output

abc abc
def abc

Regexp substitution. We can specify a regular expression as the first argument to sub() and gsub. Any regular expression metacharacters can be used here.

Ruby program that uses sub with regexp

value = "cat and dog"

# Replaced at a matching the regexp with another string.
value.sub!(/c\w\w/, "bird")
puts value

Output

bird and dog

Regexp pattern

c    The lowercase letter "c".
\w   A word character (letter or digit).

Regexp and gsub. The gsub method too can be used with a regular expression. Unlike sub(), it will replace all the matches in the string.

Important: It is often necessary to write more specific regular expressions. Here we use "\w" to prevent non-word chars from being matched.

Ruby program that uses gsub with regexp

value = "quickly, slowly or happily"

# Replace all word sending with "ly" with a string.
value.gsub!(/\w+ly/, "REP")
puts value

Output

REP, REP or REP

Regexp pattern

\w+    One or more word characters.
ly     The lowercase substring "ly".

Method block. The sub and gsub methods can be used with method blocks. Here we declare a variable, which is filled with the matched text. The right side returns a replacement.

Here: We uppercase all sequences of four word chars together with an uppercased, bracketed version.

Ruby program that uses gsub, method block

value = "bird and fish"

# Replace all four-letter words with an uppercase version.
value.gsub!(/\w{4}/) {|word| "[" + word.upcase() + "]"}
puts value

Output

[BIRD] and [FISH]

Regexp pattern

\w{4}    Four word characters.

Special replacements. We can capture groups in sub and gsub and use a special replacement code like "\1" to insert them in the result.

Tip: For multiple groups, we can use "\2" and "\3" and even further numbers. This is not demonstrated here.

Tip 2: In the string literal we use an escaped backslash character "\\1." This is needed due to a single backslash's meaning.

Ruby that uses gsub, special sequences

data = "123 456 789"

# Prefix all numbers with a number code.
# ... We must escape the backslash.
result = data.gsub(/(\d\d\d)/, "n:\\1")
puts result

Output

n:123 n:456 n:789

With sub and gsub, we have powerful substitution methods. We replace strings according to patterns. We can target just the first match, or all global matches.

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