Regular Expressions — Grouping, Quantified Repetition and Alternation

In the previous articles, we have already covered basic Notations, some Meta Characters, Character Ranges and Repetitions etc.

In case, you haven’t gone through above, I highly recommend to cover them. You can find the same using below links.

1. Getting Started With Regular Expressions.
2. Regular Expressions — Part 2
3. Regular Expressions — Part 3

Here, we will continue further topics like Grouping, Quantified repetition and Alternation etc.

Quantified Repetition

By using +, *, ? as meta character, we assure the preceding pattern will get repeated 0, 1 or infinite times. But what if I want to repeat some pattern for a specific number of times, say 3 or 4. This case is called quantified repetition.

For this we use this syntax :- {min, max}

1. Here min is the minimum number of times the pattern must be repeated
2. And max is the maximum number of times the pattern must be repeated, but its optional.

E.g.:- \d{2,4} will match numbers with min 2 and max 4 digits.

\d{2}, will match numbers with exactly 2 digits. (min and max both are equal to 2)

\d{2,}, here it will match numbers with 2 or more digits. (max in infinite)

So, for a real example, if we want to match some phone number with fixed pattern, like 123–124–6677 i.e. 3 digit followed by hyphen, followed by 3 digit followed by hyphen, and then 4 digits at the end.

We can write regex as :- /\d{3}-\d{3}-\d{4}/g

if our pattern is somewhat flexible and allows either 3 or 4 numbers in middle, then we will change the expression as:- /\d{3}-\d{3,4}-\d{4}/g

Grouping

Some times, we want to combine some pattern or character, to make a grouped expression. For e.g., if our text is like:- acd bcd abcd

And we want to match cd preceded by only ab combined, not either a or b individually. Then our regex will be :- /(ab)cd/g

Here ab will be treated a single pattern, and will be matched together everywhere.

This is called Grouping. To use this, we have to combine them between starting and closing brackets, i.e. ( and ).

For example, take this quote :- impossible itself says i am possible

Now, to match either possible or impossible, in this text, we can write regex like :- /(im)?possible/g

It will first combine im as one expression, match it either 0 or 1 time (using ?), and followed by possible.

Alternation

If we have multiple choices, to match the expression, then alternation comes handy. It is nothing but a simple OR operator i.e. |.

E.g. :- Good Morning Good Evening Good Afternoon Good Night

and we want to match either Good morning or Good night only, and not remaining.

we can write regex like :- /Good (Morning|Night)/g

For a more practical example, lets have a text like :- I like Apple Shake or Apple Juice but not Banana Shake or Fruity Juice.

Here, if we want to match only Apple things, then the regex will be like :- /Apple (Shake|Juice)/g

So, that is all for this article. We will next look about Lazy and Greedy behavior of regular expressions, in next article. Stay tuned.