<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
lexical	B-Application
grammar	I-Application
or	O
Lexical	O
Structure	O
is	O
a	O
formal	O
grammar	O
defining	O
the	O
syntax	O
of	O
tokens	O
.	O
</s>
<s>
The	O
lexical	B-Application
grammar	I-Application
lays	O
down	O
the	O
rules	O
governing	O
how	O
a	O
character	O
sequence	O
is	O
divided	O
up	O
into	O
subsequences	O
of	O
characters	O
,	O
each	O
part	O
of	O
which	O
represents	O
an	O
individual	O
token	O
.	O
</s>
<s>
This	O
is	O
frequently	O
defined	O
in	O
terms	O
of	O
regular	B-Language
expressions	I-Language
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
lexical	B-Application
grammar	I-Application
for	O
many	O
programming	O
languages	O
specifies	O
that	O
a	O
string	B-Language
literal	I-Language
starts	O
with	O
a	O
character	O
and	O
continues	O
until	O
a	O
matching	O
is	O
found	O
(	O
escaping	B-Algorithm
makes	O
this	O
more	O
complicated	O
)	O
,	O
that	O
an	O
identifier	O
is	O
an	O
alphanumeric	O
sequence	O
(	O
letters	O
and	O
digits	O
,	O
usually	O
also	O
allowing	O
underscores	O
,	O
and	O
disallowing	O
initial	O
digits	O
)	O
,	O
and	O
that	O
an	O
integer	O
literal	O
is	O
a	O
sequence	O
of	O
digits	O
.	O
</s>
<s>
Regular	B-Language
expressions	I-Language
for	O
common	O
lexical	O
rules	O
follow	O
(	O
for	O
example	O
,	O
C	O
)	O
.	O
</s>
<s>
Unescaped	O
string	B-Language
literal	I-Language
(	O
quote	O
,	O
followed	O
by	O
non-quotes	O
,	O
ending	O
in	O
a	O
quote	O
)	O
:	O
</s>
<s>
Escaped	O
string	B-Language
literal	I-Language
(	O
quote	O
,	O
followed	O
by	O
escaped	O
characters	O
or	O
non-quotes	O
,	O
ending	O
in	O
a	O
quote	O
)	O
:	O
</s>
