# Regular expression quick start guide
### Quick Reference Table
| Regular Expression Pattern | Explanations | Examples
| |
| ----------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------- | ----------- |
| Meta characters \[^$. | ?\*+(
| Special caracters used in regex.
- Must be escape with backslash "" to use a literal characters.
|
|
| Literal characters
| All characters (except the metacharacters) match a single instance of themselves.
- { and } are literal characters, unless they're part of a valid regular expression token (e.g. the {n} quantifier).
| /a/ matches "a"
| |
| \[characters]
| Character classes or character set. A character class matches a single character out of all the possibilities offered by the character class.
| /\[0-9]/ matches a single digit
| |
| \[\d]
| Shorthand character classes matching digits. Same as \[0-9].
| /\[\d]/ matches a single digit
| |
| .
| Dot matches any characters.
| /a.c/ matches both "a4c" and "ayc"
| |
| ^ | Matches at the start of the string the regex pattern is applied to. Matches a position rather than a character. |
| |
| $ | Matches at the end of the string the regex pattern is applied to. Matches a position rather than a character. |
| |
| {m,n}
| Matches at least “m” and at most “n” occurrences of preceeding character, character class or group.
|
| |
| \* | Matches zero or more occurrences of preceeding character, character class or group. |
| |
| + | Matches one or more occurrences of preceeding character, character class or group.
|
| |
| ? | Matches zero or one occurrences of preceeding character, character class or group. |
| |
| () | Parentheses are used for group or capturing group
|
| |
| \0, \1, \2, ... | Substitute the value matched by the nth grouped sub-expression, used in remapped fields. |
| |
| ?! | Not, as in "everything except this". |
| |
\
Examples
Here are some examples:
Add 2720 prefix:
```
/^(\d+)$/2720\1/
```
or
```
/^([0-9]*)$/2720\1/
```
Strip first 4 digits:
```
/^([0-9]{4})([0-9]*)$/\2/
```
Strip # and 7 first digits:
```
/^([#])([0-9]{7})([0-9]*)$/\3/
```
### Web Online Tools
* Regular builder tool (with replace) : [www.gskinner.com/RegExr](http://www.gskinner.com/RegExr)
Tips to use:
1. No need to enclose the Regular expression with '/'.
2. Replace : the regular expression must be split in two parts and '\\' replaced by '$'. For example, with '/^(\[0-9]\*)$/2720\1/', the '^(\[0-9]\*)$' would be filled on the first line and '2720$1' on the second line.
* Ruby regular expression editor and tester : [rubular.com](http://rubular.com/)
### References
* [How to Use RegEx in Remapped Called and Calling Number Mask](https://docs.telcobridges.com/wiki/Toolpack:_How_to_Use_RegEx_in_Remapped_Called_and_Calling_Number_Mask)
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