Getting Started with Regular Expressions

cjx is currently working on a crawler project and urgently needs to capture the content he wants from the page, but obtaining it through logical judgment is too complicated. Fortunately, we have the powerful tool of regular expressions. Many things can be done easily. Cjx also had some knowledge about regular expressions before. However, I have always been in a state of incomplete understanding, and it is difficult to effectively write a satisfactory regular expression by myself. Recently I found a book on the Internet called Mastering Regular Expressions written by Jeffrey E.F. Fried. After reading the first chapter, I suddenly found that I could write a few regular rules, haha~~~ cjx suddenly felt like he had upgraded from a loser to a tall, rich and handsome man... The following is a summary of the first chapter of the book~

Start and end of line

Perhaps the easiest to understand metacharacters are the caret symbol ^ and the dollar sign $. When checking a line of text, ^ represents the beginning of a line and $ represents the end.

Readers should develop the habit of understanding regular expressions according to characters. For example, don’t do this:

　^cat matches lines starting with cat

but should be understood like this:

　^cat matches text that starts with c as the first character of a line, followed by an a, followed by a t .

There is no difference in the results of these two interpretations, but it is easier to understand the internal logic of newly encountered regular expressions by interpreting them by characters.

Match one of several characters

If we need to search for the word "grey" and are not sure whether it is written as "gray", we can use the regular expression structure [...]. It allows the user to list characters that are expected to match somewhere, usually called a character group.

So, gr[ea]y means: find g first, followed by an r, then an a or e, and finally a y.

Within the character group, the character metacharacter '-' represents a range: is exactly the same as . We can also combine character ranges with normal text as we like:

[0-9A-Z_!.?] can match a number, uppercase letter, underscore, exclamation point, period or question mark.

Exclusive character group

Replace [...] with [^...]. This character group will match any unlisted characters. For example: [^1-6] matches any character except 1 to 6. The ^ at the beginning of this group means exclusion, so what is listed here is not the characters you want to match, but the characters you don't want to match.

Use dot to match any character

metacharacter. It is a simple way to write a character group used to match any character. If we need to use a "match any character" placeholder in an expression, using the dot is convenient.

Match any subexpression

The metacharacter | is a very concise metacharacter, which means "or". Relying on it, we can combine different sub-expressions into a total expression, and this total expression can match any sub-expression.

Optional elements

Now let’s look at the matching of color and colour. The difference between them is that the following word has one more u than the previous one. We can use coloru?r to solve this problem. The metacharacter ? (that is, the question mark) represents optional options. Adding it after a character means that the character is allowed to appear here, but its occurrence is not a necessary condition for successful matching.

Other quantifiers: Repeated

+ (plus sign) and * (asterisk) function similarly to question marks. The metacharacter + means that the immediately preceding element appears one or more times, while * means that the immediately preceding element appears any number of times, or does not appear at all.
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