Regular Expressions (Regexp)

Theory: Quantification

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In this lesson, we will look at quantification and observe how to find repeating characters using it.

Quantification is searching for sequences. A quantifier is a notation that specifies the number of possible repetitions of a character, a group of characters, or a character class in a regular expression before it.

We will see what all this means. Look at an example with the simplest quantifier ?, which means to search for matches repeating from zero to one time:

/colou?/

colr, color, colour, colouur, colouuur

There is no grouping or character class in this expression. So, the quantifier ? specifies the number of repetitions for the character u. It means the preceding character u will repeat ether:

Zero times — it does not repeat at all
One time — it occurs once without repetitions

The result is four matches.

And in this example, we will add the character r to the pattern string. It will only give us two matches:

/colou?r/

colr, color, colour, colouur, colouuur

Using a grouping and a character class, we get different matches:

With grouping, we check for the occurrence of the whole group for zero or one time
With character classes, we check if one of the characters enters for zero or one time. For this case, we don't examine all characters simultaneously

/col(ou)?r/

colr, color, colour, colouur, colouuur

/col[ou]?r/

colr, color, colour, colouur, colouuur

Another often-used quantifier is the + character. It indicates that we must find the preceding character, group, or class of characters at least once. It is what happens. Here, the word color is no longer matched:

/colou+r/

colr, color, colour, colouur, colouuur

The character * indicates either no repetition or multiple repetitions, giving us a match in all substrings:

/colou*r/

colr, color, colour, colouur, colouuur

There are also more specific quantifiers — we write them in curly brackets {}. Between them, you enter the number of repetitions you need:

/colou{2}r/

colr, color, colour, colouur, colouuur

In addition, you can enter a range of repetitions in curly brackets {}. For example, from two to three:

/colou{2,3}r/

colr, color, colour, colouur, colouuur

If we don't enter a number for the upper bound of the range, there won't be a maximum number of repetitions: