To learn the regular expression, understanding meta-characters is a must to overcome the difficulties.
Don't try to remember.
.: matches any single character.
For example, the regular expression "B.G" can match the following string: "Big", "Bug", "BG", but does not match "Buug", "B." G "Can match" Buug ". []: Matches any one of the characters in the parentheses.
For example, the regular expression "b[aui]g" matches the bug, big and bag, but does not match beg, Baug. You can use the hyphen "-" in parentheses to specify the interval of the character to simplify the representation, such as the regular expression [0-9] can match any numeric character, so that the regular expression "a[0-9]c" equivalent to "a[0123456789]c" can Match "a0c", "A1c", "A2C" such as String, you can also create multiple intervals, such as "[A-za-z]" can match any uppercase and lowercase letters, "[a-za-z0-9]" can match any uppercase or lowercase letters or numbers. (): The expression that is enclosed in () is defined as "group", and the character that matches the expression is saved to a staging area, which is useful when the string is extracted. To represent some characters as a whole. Change the priority, define the extraction group of two roles. : A logical OR operation of two matching criteria.
' Z|food ' can match "z" or "food". ' (z|f) Ood ' matches "Zood" or "food".
*: Match 0 to more sub-expressions before it, and wildcards * okay.
For example, the regular expression "zo*" can Match "Z", "Zo" and "Zoo", so ". *" means that you can match any string. "Z (b|c) *" →zb, ZBC, ZCB, ZCCC, ZBBBCCC. "Z (AB) *" can match Z, Zab, Zabab (with parentheses to change precedence). +: Matches the preceding subexpression one or more times, and * contrasts (0 to multiple).
For example, the regular expression + + matches 9, 99, 999, and so on. "zo+" can Match "Zo" and "Zoo" and cannot match "Z".? : matches the preceding subexpression 0 or one time.
For example, "Do (es)?" You can match "do" or "does". Typically used to match the "optional section". {n}: matches the determined n times.
"Zo{2}" →zoo. For example, "e{2}" cannot match "E" in "bed", but can match two "E" in "seed".
{N,}: matches at least n times.
For example, "e{2,}" cannot match "E" in "bed", but can match all "E" in "Seeeeeeeed". {n,m}: matches at least n times and matches up to M times.
"e{1,3}" will match the first three "E" in "Seeeeeeeed"
^ (shift+6): Matches the start of a row.
For example, the regular expression "^regex" can match the beginning of the string "Regex I will use", but does not match "I will use regex". ^ Another meaning: No! (not understood at the moment) $: Match line terminator.
For example, the regular expression "cloud $" can match the string "Everything is a cloud" end, but cannot match the string "floating clouds Ah"
Regular Expressions--metacharacters
