useful sites
characters that must be escaped:
[, \, ^, $, ., |, ?, *, +, (, )
character matches
\d |
single character that is a digit |
\n |
line feed (0x0A) |
\r |
carriage return (0x0D) |
\s |
whitespace character (includes tabs and line breaks) |
\t |
tab character (ASCII 0x09) |
\w |
“word character” (alphanumeric characters plus underscore) |
. |
single character, except line break characters. It is short for [^\n] (UNIX regex flavors) or [^\r\n] (Windows regex flavors) |
[] |
match a character class |
- |
inside a character class to specify a range of characters |
^ |
after the opening square bracket will negate the character class |
*remember that Windows text files use \r\n to terminate lines, while UNIX text files use \n.
*if your regular expression engine supports Unicode, use \uFFFF to insert a Unicode character. E.g. \u20AC matches the euro currency sign.
anchors
(anchors do not match any characters. they match a position.)
^ |
matches at the start of the string |
$ |
matches at the end of the string |
\b |
matches at a word boundary. A word boundary is a position between a character that can be matched by \w and a character that cannot be matched by \w |
\B |
matches at every position where \b cannot match. |
other matches
| |
or |
? |
makes the preceding token in the regular expression optional E.g.: colou?r matches colour or color. |
* |
attempt to match the preceding token zero or more times |
+ |
attempt to match the preceding token once or more E.g.: <[A-Za-z][A-Za-z0-9]*> matches an HTML tag without any attributes. <[A-Za-z0-9]+> is easier to write but matches invalid tEleven characters with special meanings: the opening square bracket [, the backslash \, the caret ^, the dollar sign $, the period or dot ., the vertical bar or pipe symbol |, the question mark ?, the asterisk or star *, the plus sign +, the opening round bracket ( and the closing round bracket ). These special characters are often called “metacharacters”.ags such as <1>. |
{} |
specify amount of repetition E.g.: Use \b[1-9][0-9]{3}\b to match a number between 1000 and 9999. \b[1-9][0-9]{2,4}\b matches a number between 100 and 99999.Use {3,} to match 3 or more repetitions |
() |
create a group |
\# |
match a group (use slash plus a number, starting at 1, to indicate which group, i.e. \1 for the first group |
(?i) |
case insensitive regex |
(?=pattern) |
zero-width positive look-ahead assertion. For example, /\w+(?=\t)/ matches a word followed by a tab, without including the tab. |
matches that must be escaped in vim:
+ ( ) |
examples:
q[^x] matches qu in question. It does not match Iraq since there is no character after the q for the negated character class to match.
\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b
search for an email address
^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$
verify properly formatted email address
:%s/^\s\+\(o\|*\|+\)\s//g
(vim) remove whitespace followed by one of o, *, +, and one more whitespace character at beginning of line (for copying stuff from Google docs to remove formatting)