Regex to match less than a two-digit number

Question

I have some records as such, in a file:

A 20 year old programmer
A 52 year old politician
A 12 year old practitioner
Many many more of these...

I want to match only lines that contain a number less than 20. I have tried:

^[0-20]{2}$

But it works for only numbers 0-2. How should I construct a regular expression to match numbers < 20? For instance, it should match:

A 12 year old practitioner

But not

A 20 year old programmer
A 52 year old politician

score 3 · Accepted Answer · answered Nov 15 '18 at 08:03

3

You may use

\b1?[0-9]\b

Details

Word boundary variations

To match anywhere in a string, even if glued to a word:

(?<!\d)1?[0-9](?!\d)

To only match in between whitespaces:

(?<!\S)1?[0-9](?!\S)

answered Nov 15 '18 at 08:03

Wiktor Stribiżew

1

That "glued to a word" one is exactly what I want, thanks so much! I'll accept this when the grace period ends. – Jack Bashford Nov 15 '18 at 08:05
@JackBashford Great. Note that in case you want to also match `00` you need to replace `1?` with `[01]?` – Wiktor Stribiżew Nov 15 '18 at 08:06
Ahh, because that would "optionally match a `0` or a `1`"? – Jack Bashford Nov 15 '18 at 08:07
@JackBashford `[01]` is a [**character class**](https://www.regular-expressions.info/charclass.html) matching any single char defined inside the class, and `?` is a greedy quantifier matching the quantified pattern 1 or 0 times. – Wiktor Stribiżew Nov 15 '18 at 08:09
1

@JackBashford Also, see [Why doesn't \[01-12\] range work as expected?](https://stackoverflow.com/questions/3148240/why-doesnt-01-12-range-work-as-expected) – Wiktor Stribiżew Nov 15 '18 at 08:10

score 1 · Answer 2 · answered Nov 15 '18 at 08:03

Using regex to match digit ranges is usually a bit clumsy, but here, you can do it pretty simply with:

\b1?\d\b

In plain language: an optional one, followed by a digit. So, any standalone digit is allowed, but a two-digit number needs its first digit to be a 1.

If you want to permit leading zeros, change to \b[01]?\d\b.

2 Answers2