21  21. The complete list of quantifiers

21.1 The complete list of quantifiers

# The complete list of quantifiers 
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# In general you can use ? to turn any greedy quantifier into a non-greedy
# quantifier
#
#   *  - zero or more (greedy)
#   *? - zero or more (non-greedy)
#
#   +  - one or more (greedy)
#   +? - one or more (non-greedy)
#
#   ?  - zero or one (greedy)
#   ?? - zero or one (non-greedy)
#
# NOTE the following also allow for non-greedy ? modifier. However, these
# are not really necessary - see the notes below.
#
#   {3,5}  - 3,4 or 5 repetitions (greedy - i.e. will match all 5 if they are there)
#   {3,5}? - (non-greedy - will match 3 even if there are 5)
#            Notice - you could just write {3} instead of {3,5}? (think about it)
#
#   {3,}   - (greedy)     3 or more matches in a row, matches as many as there are
#   {3,}?  - (non-greedy) will always match first 3 even if there are more (non-greedy)
#            Notice - you could just write {3} instead of {3,}? (think about it)
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

21.1.1 examples

# EXAMPLE
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

quotedCsv = c('"a,b,c","apple,orange","watermellon"')
cat(quotedCsv)

"a,b,c","apple,orange","watermellon"

# The following gsub uses a greedy quantifier, ie. *.
# It will match as much as it can.

# greedy
gsub('".*"', 'QUOTES', quotedCsv)   # "QUOTES"

[1] "QUOTES"

# non-greedy
gsub('".*?"', 'QUOTES', quotedCsv) # "QUOTES,QUOTES,QUOTES"

[1] "QUOTES,QUOTES,QUOTES"

# A MORE COMPLEX EXAMPLE
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

text = "She sells sea shells by the sea shore."

result = gsub("(.*)(sea)(.*)", "1st \\1\n2nd \\2\n3rd \\3", text)

cat(result)

1st She sells sea shells by the 
2nd sea
3rd  shore.

# Reults are "greedy", i.e. the .* in the beginning matches as much as 
# it can as long as the whole regex will work. The result is:

# 1st part: She sells sea shells by the 
# 2nd part: sea
# 3rd part: shore.

# The following DOES NOT happen
#
# 1st part:    She sells 
# 2nd part:    sea
# 3rd par:     shells by the sea shore.


# we can make the regex UN-GREEDY by using a ? AFTER the *
#

text = "She sells sea shells by the sea shore."


result = gsub("(.*?)(sea)(.*)", "1st \\1\n2nd \\2\n3rd \\3", text)

cat(result)

1st She sells 
2nd sea
3rd  shells by the sea shore.