Regular expression stepping on the pit

1. As long as the /g attribute is added to the regular expression, you need to pay attention to the change of the reg.lastIndex attribute value when executing the RegExp API

RegExp.exec(string)

Perform a search match in a specified string

exec(string): array || null

parameter

string

return value

• Match successfully

  • Returns an array, the 0th element of this array is the text that matches the regular expression, the first element is the text that matches the first sub-expression of RegExpObject (if any), and the second element is The text that matches the second sub-expression of RegExpObject (if any), and so on;
  • In addition to the array elements and the length attribute, the exec() method also returns two attributes. The index attribute declares the position of the first character of the matched text. The input attribute stores the retrieved string string;
  • RegExp does not have when global or sticky flag is set, reg.lastIndex is 0;
  • When the global or sticky flag is set in RegExp, reg.lastIndex points to the next start position, which means that the exec method can be called repeatedly to traverse all matching text in the string; when exec() can no longer find a matching text , It will return null and reset the lastIndex property to 0
• Match failed
  Return null

Precautions

• After the RegExp object has set the global or sticky flag, the RegExp object will have a state (the position after the last successful match will be recorded in the lastIndex property);
• After the RegExp object has set the global or sticky flag, if a new string is to be retrieved after a pattern match is completed in a string, the lastIndex property must be manually reset to 0.

demo

const regex1 = RegExp("foo*", "g");
const str1 = "table football, foosball";
let array1;

while ((array1 = regex1.exec(str1)) !== null) {
  console.log(`Found ${array1[0]}. Next starts at ${regex1.lastIndex}.`);
  // expected output: "Found foo. Next starts at 9."
  // expected output: "Found foo. Next starts at 19."
}

refer to

w3c exec
MDN exec

RegExp.test(string)

Used to check whether the regular expression matches the specified string

test(string): bool

parameter

string

return value

bool , match successfully true ; fail false ;

Precautions

• After the RegExp object has set the global or sticky flag, the RegExp object will have a state (the position after the last successful match will be recorded in the lastIndex property);
• After the RegExp object has set the global or sticky flag, if a new string is to be retrieved after a pattern match is completed in a string, the lastIndex property must be manually reset to 0.
• As long as there is a partial match, it will return true, you need to add ^ $

refer to

MDN test

Add ^ $ before and after

RegExp.exec(string) and RegExp.test(string) matters needing attention

When using the RegExp object with the global flag set, you must pay attention to the change of the value of lastIndex; pay attention to modify the lastIndex as needed (for example, the same regular, after matching the A string, then matching the B string)

string.match(RegExp)

Retrieval returns the result of a string matching a regular expression

string.match(RegExp):array || null

parameter

RegExp

return value

• Match successfully

  • Array, RegExp with g global flag, returns all matches
  • Array, RegExp does not have the g global flag, and returns the first match
• Match incident
  null

string.match(RegExp) and RegExp.exec(string)

If the regular expression does not contain the g flag, str.match() will return the same result as RegExp.exec().