metacharacters

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


[] => Square brackets. Square brackets specifies a set of characters you wish to match.
       [abc] will match to a, ab, abc, ac, bc, ca, ba. 
       [a-e] => same as [abcde]
       [1-5] => same as [12345]
       [0-39] => same as [01239]
       [^abc] means any character except a or b or c.
       [^0-9] means any non-digit character.


. => Period. A period matches any single character (except newline '\n')
        .. will match a, ac, acd, acde, etc.

^ => Caret. The caret symbol ^ is used to check if a string starts with a certain character.
        ^a will match a, ab, acd, abs, afgr, etc.

$ => Dollar. The dollar symbol $ is used to check if a string ends with a certain character.
        a$ will match a, formula, etc.

* => Star. The star symbol * matches zero or more occurrences of the pattern left to it.
        ma*n will match mn, man, maaan, woman, etc.

+ => Plus. The plus symbol + matches one or more occurrences of the pattern left to it.
        ma+n will match man, maaan, woman

? => Question. The question mark symbol ? matches zero or one occurrence of the pattern left to it.
        ma?n will match mn, man, woman

{} => Braces. Consider this code: {n,m}. This means at least n, and at most m repetitions of the pattern left to it.
        a{2, 3} will match abs daat, aabc daaat, aabc daaaat

| => Alternation. Vertical bar | is used for alternation (or operator).
        a|b will match ade, acdbea, etc.

() => Group. Parentheses () is used to group sub-patterns. For example, (a|b|c)xz match any string that matches either a or b or c followed by xz
        (a|b|c)xz will match abxz, axz cabxz

\ - Backslash. Backlash \ is used to escape various characters including all metacharacters