Regular expression character cluster (2)

Identify recurring occurrences

By now, you already know how to match a letter or number, but more often than not, you may want to match a word or a group of numbers. A word consists of several letters, and a group of numbers consists of several singular numbers. The curly braces ({}) following a character or character cluster are used to determine the number of times the preceding content is repeated.

Character cluster meaning

^[a-zA-Z_]$ All letters and underscores

^[[:alpha:]]{ 3}$ All 3-letter words

^a$ Letter a

^a{4}$ aaaa

^a{2,4}$ aa, aaa or aaaa

^a{1,3}$ a,aa or aaa

^a{2,}$ A string containing more than two a's

^a{2,} Such as: aardvark and aaab, but not apple

a{2,} Such as: baad and aaa, but not Nantucket

\t{2} Two tabs Symbols

.{2} All two characters

These examples describe three different uses of curly braces. A number, {x} means "the preceding character or character cluster appears only x times"; a number plus a comma, {x,} means "the preceding content appears x or more times"; two Comma-separated numbers, {x,y} means "the previous 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 underscore

^[0-9]{1,}$ //All positive numbers

^\-{0,1}[0-9]{1,}$ //All Integer

^\-{0,1}[0-9]{0,}\.{0,1}[0-9]{0,}$ //All decimals

The last example is not easy to understand, is it? Look at it this way: with everything starting with an optional negative sign (\-{0,1}) (^), followed by 0 or more digits ([0-9]{0,}), and an optional The chosen decimal point (\.{0,1}) followed by 0 or more digits ([0-9]{0,}) and nothing else ($). Below you will learn about the simpler methods you can use.

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

　 ^\-?[0-9]{0,}\.?[0-9]{0,}$

Special characters”* " is equal to {0,}, they both represent "0 or more previous contents". Finally, the character "+" is equal to {1,}, which means "one or more previous contents", so the above 4 examples can be written as:

^[a-zA-Z0- 9_]+$ //All strings containing more than one letter, number or underscore

^[0-9]+$ //All positive numbers

^\-?[ 0-9]+$ //All integers

^\-?[0-9]*\.?[0-9]*$ //All decimals

Of course this It doesn't technically reduce the complexity of regular expressions, but it makes them easier to read.

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