Ch4 Regular expression / SQL

Character Classes¶

Class	Description	Matched Example	Unmatched Example
`[abc]`	Matches a, b, or c	a
`[a-z]`	Matches any character between a and z	x
`[^A-Z]`	Matches any character that is not between A and Z.	a
`\w`	Matches any "word" character. Equivalent to `[A-Za-z0-9_]`.	A, a, 0, _	- + * / ......
`\d`	Matches any digit. Equivalent to `[0-9]`.
`[0-9]`	Matches a single digit in the range 0 - 9. Equivalent to `\d`.	1
`\s`	Matches any whitespace character (spaces, tabs, line breaks).
`.`	Matches any character besides new line.

Character classes can be combined, like in [a-zA-Z0-9].

Combining Patterns¶

There are multiple ways to combine patterns together in regular expressions.

Combo	Description
`AB`	A match for A followed immediately by one for B. Example: `x[.,]y` matches "x.y" or "x,y".
`A\|B`	Matches either A or B. Example: `\d+\|Inf` matches either a sequence containing 1 or more digits or "Inf".

A pattern can be followed by one of these quantifiers to specify how many instances of the pattern can occur.

Symbol	Description
`*`	0 or more occurrences of the preceding pattern. Example: `[a-z]*` matches any sequence of lower-case letters or the empty string.
`+`	1 or more occurrences of the preceding pattern. Example: `\d+` matches any non-empty sequence of digits.
`?`	0 or 1 occurrences of the preceding pattern. Example: `[-+]?` matches an optional sign.
`{1,3}`	Matches the specified quantity of the preceding pattern. `{1,3}` will match from 1 to 3 instances. `{3}` will match exactly 3 instances. `{3,}` will match 3 or more instances. Example: `\d{5,6}` matches either 5 or 6 digit numbers.

Groups¶

圆括号用于创建groups，和平常的算术表达式中的圆括号类似。 For example, (Mahna)+ matches strings with 1 or more "Mahna", like "MahnaMahna". Without the parentheses, Mahna+ would match strings with "Mahn" followed by 1 or more "a" characters, like "Mahnaaaa".

Anchors¶

^: Matches the beginning of a string. Example: ^(I|You) matches I or You at the start of a string.
$: Normally matches the empty string at the end of a string or just before a newline at the end of a string. Example: (\.edu|\.org|\.com)$ matches .edu, .org, or .com at the end of a string.
\b: Matches a "word boundary", the beginning or end of a word. Example: s\b matches s characters at the end of words, \bs matches s characters at the beginning of words.

Special Characters¶

The following special characters are used above to denote types of patterns:

\ / ( ) [ ] { } + * ? | $ ^ .

That means if you actually want to match one of those characters, you have to escape it using a backslash. For example, $1\+3$ matches "(1 + 3)".