Regular expressions in PHP (2)

Regular expressions in PHP (2). Read the regular expressions in PHP (2). Make sure that the regular expressions appear until now. You already know how to match a letter or number, however, in more cases, you may need to match a word or a group of numbers. A word may consist of several letters, and a group of numbers may consist of several singular numbers. Curly braces following the character or character cluster ({"> <LINKhref =" the http://www.php100.co is sure to repeat

Until now, you know how to match a letter or number, but in more cases, you may need to match a word or a group of numbers. A word may consist of several letters, and a group of numbers may consist of several singular numbers. Braces ({}) following the character or character cluster are used to determine the number of occurrences of the preceding content.

Character cluster meaning
^ [A-zA-Z _] $ all letters and underscores
^ [[: Alpha:] {3} $ all 3-letter words
^ A $ letter
^ A {4} $ aaaa
^ A {2, 4} $ aa, aaa or aaaa
^ A {1, 3} $ a, aa or aaa
^ A {2, }$ contains more than two a Strings
^ A {2,} for example, aardvark and aaab, but not apple
A {2,} such as baad and aaa, but not Nantucket
\ T {2} two tabs
. {2} all two characters

These examples describe three different usages of curly brackets. A number, {x} indicates "the character or character cluster appears only x times"; a number is added with a comma, {x ,} "The preceding content appears x or more times"; two numbers separated by commas (,). {x, y} indicates that "the preceding content appears at least x times, but not more than y times ". We can extend the pattern to more words or numbers:

^ [A-zA-Z0-9 _] {1, }$ // All strings containing more than one letter, number, or underline
^ [0-9] {1, }$ // all positive numbers
^ \-{0, 1} [0-9] {1, }$ // all integers
^ \-{0, 1} [0-9] {0 ,}\. {0, 1} [0-9] {0 ,}$ // all decimals

The last example is hard to understand, right? Let's take a look: It starts with an optional negative sign (\-{0, 1}) (^), followed by 0 or more numbers ([0-9] {0,}), and an optional decimal point (\. {0, 1}) followed by 0 or multiple numbers ([0-9] {0,}), and nothing else ($ ). Next you will know the simpler method that can be used.

Special character "? "It is equal to {0, 1}, and both represent" 0 or 1 previous content "or" previous content is optional ". So the example just now can be simplified:

^ \-? [0-9] {0 ,}\.? [0-9] {0,} $

The special characters "*" and {0,} are equal. They all represent "0 or multiple front content ". Finally, the character "+" is equal to {1,}, indicating "one or more previous content". Therefore, the preceding four examples can be written as follows:

^ [A-zA-Z0-9 _] + $ // all strings that contain more than one letter, number, or underline
^ [0-9] + $ // all positive numbers
^ \-? [0-9] + $ // all integers
^ \-? [0-9] * \.? [0-9] * $ // all decimals

Of course, this does not technically reduce the complexity of regular expressions, but it can make them easier to read.