1-20
Character
Meaning
Remarks
string
$
Ending sign,
string
appears only
at the end of a line.
For example, regular expression "user$”
only matches a string ending with “user”,
not “userA”.
.
Full stop, a wildcard used in
place of any character,
including single character,
special character and blank.
For example, “.l” can match “vlan” or
“mpls”.
*
Asterisk, used to match a
character or character group
before it zero or multiple times.
For example, “zo*” can match “z” and
“zoo”; (zo)* can match “zo” and “zozo”.
+
Addition, used to match a
character or character group
one or multiple times before it
For example, “zo+” can match “zo” and
“zoo”, but not “z”.
|
Vertical bar, used to match the
whole string on the left or right
of it
For example, “def|int” can only match a
character string containing “def” or “int”.
_
Underline. If it is at the
beginning or the end of a
regular expression, it equals ^
or $; in other cases, it equals
comma, space, round bracket,
or curly bracket.
For example, “a_b” can match “a b” or
“a(b”; “_ab” can only match a line
starting with “ab”; “ab_” can only match a
line ending with “ab”.
-
Hyphen. It connects two values
(the smaller one before it and
the bigger one after it) to
indicate a range together with
[ ].
For example, “1-9” means numbers from
1 to 9 (inclusive); “a-h” means from a to h
(inclusive).
[ ]
A range of characters, Matches
any character in the specified
range.
For example, [16A] can match a string
containing any character among 1, 6,
and A; [1-36A] can match a string
containing any character among 1, 2, 3,
6, and A (with - being a hyphen).
“]” can be matched only when it is put at
the beginning of [ ] if it is used as a
common character in [ ], for example
[ ]
string
]. There is no such limit on “[”.
( )
A character group. It is usually
used with “+” or “*”.
For example, (123A) means a character
group “123A”; “408(12)+” can match
40812 or 408121212. But it cannot
match 408.
\index
Repeats a specified character
group for once. A character
group refers to the string in ()
before \.
index
refers to the
sequence number (starting from
1 from left to right) of the
character group before \: if only
one character group appears
before \, then
index
can only be
1; if n character groups appear
before
index
, then
index
can be
any integer from 1 to n.
For example, (
string
)\1 means to repeat
string
for once, and (
string
)\1 must
match a string containing
stringstring;
(
string1
)(
string2
)\2 means to repeat
string2
for once, and (
string1
)(
string2
)\2
must match a string containing
string1string2string2;
(
string1
)(
string2
)\1\2 means to repeat
string1
for once first
,
and then repeat
string2 for once
, and
(
string1
)(
string2
)\1\2 must match a string
containing
string1string2string1string2.
