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So let's take a look at how we use regular expressions in Python.

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We'll start by making a working copy of regex.py and we'll call

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this regex-working.py.

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Regex is a common short name for regular expressions, because who wants to say

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all those syllables when you can just say regex?

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Here we have a very simple program that opens a file called raven.txt, which has

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the complete text of Edgar Allan Poe's poem, The Raven.

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So we open that file and then we read the file line-by-line and we do a regex

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search for anything that matches this pattern here, and this pattern here

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basically matches the word Lenore or the word Nevermore, which occur with some

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regularity in this poem.

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And then it prints them out and you will notice we have end= the blank string,

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because we don't want to put a new line at the end of a line that already has a

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new line at the end of it.

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And so we'll go ahead and run this and there we have the lines that have

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Nevermore or Lenore in them.

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So searching with regular expressions is done using the re.search method of the

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re module and here is our regular expression pattern.

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This is a very simple regular expression pattern.

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It's beyond the scope of this course to teach you regular expressions, but in a

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nutshell what this one does is it's an alternation and so it has the regular

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expression alternation operator which you can just call OR.

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So anything that has Len or Neverm immediately followed by the letters ore.

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So this will match Lenore and it will also match Nevermore and we see in our

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results down here that we have lines that have Nevermore, quoth the raven, and

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we have lines that have Lenore.

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So that's how you do a search using regular expressions in Python.

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If you want to, you can just print out the part of this that was matched.

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If we take this line here and instead of putting it in the if, we take its

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result and call it match, then we get a match object which we can use and this

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is some of the beauty of how Python's "everything is an object" helps us out.

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We can test match, if we have a match then what we can print here is

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match.group(), like that.

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So if I save this and run it, now we get all of the words that we actually matched.

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Now this can be really useful for simply looking for specific patterns in a set

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of text and it can also be used when we look later on for replacing patterns in

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text, when we talk about replacing later on in this chapter.

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So those are some simple examples of how you use Python's regular expression engine.