You actually now know everything you need to use very basic regular expressions. You know how to match all of the following things:

characters

Do this by just typing the chars, and using \ to escape regex symbols you want to match.

any char

Use the . (dot) to match any one char.

char sets

Use [] to make a set, including ranges of characters to match like [0-9]

inverted sets

Put a ^ inside a set and it inverts: [^a-z].

beginning anchor

A ^ at the beginning of a regex says to "anchor" it so that it only matches at the beginning.

ending anchor

A $ at the end of a regex anchors to the end so it only matches at the end. Combine with ^ to match exactly.

optional modifier

Put a ? after a regex symbol, char, or set and it will make that thing optionally matched.

one-or-more

A + after a regex symbol, char, or set and it will match one-or-more of them.

zero-or-more

A * after a regex symbol, char, or set and it's optionall there or repeated (zero or more).

Mostly everything after this is additional ways to do the following to the above concepts:

• Limit repetition in more complex ways.
• Group expressions.
• Alternate between one or more expressions.
• Special regex engine modifications for more complex tasks.

In this exercise you'll learn to limit repetition in different ways by specifying the "greediness" of the expression. To learn this we'll try matching various dates in different formats, but try to be as exact as possible.

NOV-10-2011 August 20, 2011 10-18-2011

Once you have that corpus text written out, here's the regex:

[A-Z]{3}-[0-9]{2}-[0-9]{4}
[A-Za-z]+? [0-9]{2}, [0-9]{2,4}?
[0-9]{2}-[0-9]{2}-[0-9]{2,4}?

I'm using three new kinds of syntax that all do the same thing:

• If you put {X} in the same place you'd put a + (after something to repeat) then it will make sure that it is repeated X times.
• If you put {X,Y} then it will make sure it repeates at least X times, but not more than Y times.
• If you put a ? after any of the repetition symbols it means "non-greedy". This is unfortunate because ? also means "optional" but just remember that placing it after repetition means "non-greedy".

The way to explain "non-greedy" is simply that most regex engines try to match repetition to the biggest part of the corpus text possible. If you run into situations where you're matching too much with repetition, then you can use ? to tell it to match the smallest repetition possible.

I can then break down each of the symbols I used in this regex file:

ex10.regex:1

In this regex I use {X} to limit the month to 3 characters ([A-Z]{3}), the day to 2 digits ([0-9]{2}), and the year to 4 digits ([0-9]{4}).

ex10.regex:2

This one I'm using the +? to match as many characters for the month as possible, but to make it the smallest match possible. Remember the ? after the + does this. I also use {2,4}? at the end to match 2 to 4 characters for the year, but the ? will make it match the smallest.

ex10.regex:3

Finally I'm mixing these up to match 2 or 4 elements of the date.

That file doesn't exist.
> [A-Z]{3}-[0-9]{2}-[0-9]{4}
Input file is empty. Use !load to load something.
> [A-Za-z]+? [0-9]{2}, [0-9]{2,4}?
Input file is empty. Use !load to load something.
> [0-9]{2}-[0-9]{2}-[0-9]{2,4}?
Input file is empty. Use !load to load something.
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