10: Find and Replace

Here is Textpad's 'find and replace' box. 

We open it by using F8.

You'll notice that most of the options are the same as ''find' - see post 7 for an explanation of these. There are some differences: 

No wrapped search. 'Find next' only works downwards to the end of the file.

Selected text. You can highlight some text, and make changes in the highlighted text only. 

What we're interested in here are the Go buttons. Once you've typed what you want to find into the 'Find' box, choose between:

Find next Finds the next occurence of what's in the 'find what' box

Replace Replaces the current selection with what's in the 'replace with' box

Replace next Replaces the next match of the 'find what' box' with what's in the 'replace with' box

Replace all Replaces every single match of the 'find what' box' with what's in the 'replace with' box

You will almost always use Replace all.

7: \n and \t

Here are two codes that you will use all the time:

\t means tab

\n means new line

Textpad is at its best when using tab seperated values (TSV), which is exactly what we find in excel spreadsheets. Below, you'll see data represented in a spreadsheet...

 

...and in textpad...

...and, finally, here is the first line of data as a 'find' line in textpad:

david nicholls\ttheunderstudy\tHodder & Stoughton\t1\n

So, wherever there is a tab, we represent it as \t. 

(Strictly speaking, there is enough detail here that the \n is uneccesary - it's just there to demonstrate. Another more accurate way to represent this is to use $ at the end of the line - it means 'end of the line'. More on this later.)

6: Escaping the metacharacter

So: the obvious question. The question is 'but if I write a regular expression for some text, and there's a question mark in it, and it really means 'this is the end of the sentence and the sentence is a question', what do I do?'

Easy peasy. If the symbol has a special meaning that you don't want to use, you need to escape the special meaning. To do this, you use a metacharacter. 

Every time you mean to use a symbol as a character (i.e. not a metacharacter), you use this: \

So if you had this piece of text:

What is wrong with this damn computer?

And you wanted the ? to mean (the way it usually does) 'this is the end of the sentence and the sentence is a question', you would start your regex like this: 

find: What is wrong with this damn computer\?

The \ means 'literally'. It means 'this is not a metacharacter, it's just a nice normal character. I use \ at least once in every regex I ever write.

Warning: sometimes \ reverses, and indicates that the next character is, in fact, special (I know, it's confusing) The most frequently used examples of this are \n and \t. \n means 'new line', and \t means 'tab'. More on these two in lesson seven.

 

4. Regular expressions: introduction

'Regular expressions' is a difficult name for a useful tool. Basically, they're very small computer programs. A single regular expression (or 'regex') is a little sequence of special characters that match a pattern in a file.

What we're dealing with here is 'find' and 'replace'.

Consider this little piece of text:

I have a cat because I like cats. I can never catch my cat.

If you wanted to change that to 'dog', in Excel, or Word, you could. You find 'cat' and replace it with 'dog'.

Find: cat

Replace: dog

Output: I have a dog because I like cats. I can never catch my dog.

Excel and Word will skip the word 'cats'. These programs know that you're unlikely to want to change every sequence of c-a-t, so they assume you mean "find 'cat', but only when it's definitely one word and not part of another word, like 'catch' or 'replicate' or 'subcategory'". Fair enough. Easy, too - after all, you can just do a second find-replace for 'cats'.

Textpad isn't that generous. It assumes that you mean exactly what you say, every time, with no exceptions.

In textpad, this would happen:

Find: cat

Replace: dog

Output: I have a dog because I like dogs. I can never dogch my dog.

The obvious response to that example is: That's really stupid.

Absolutely it's stupid. But here's the thing: Finding 'cat' and replacing it with 'dog' is not a regex. (As a general rule, the less text there is in a regex, the better). Regexes are not about text, they are about patterns, and the patterns are expressed in metacharacters.

A metacharacter is a single keyboard-stroke that refers to a type of character, not the character itself.

Here are some examples:

. means 'any character'

? means 'zero or one of the character before the ?

Don't bother trying to understand the next bit. The point of this example is not to show how it's done, only that it can be done. (The red is just to show which bits are doing the work).

Find: cat\(s?\)\>

Replace: dog\1

Output: I have a dog because I like dogs. I can never catch my dog.

And there we go. All the changes have been made accurately and cleanly in one move.