Lookahead, Lookbehind, Lookaround

/jsRegex Panels/i

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1. Understanding Lookaround
Word Wrap
Positive Lookahead /x(?=y)/ matches a pattern (x) on condition that it is followed by another pattern (y)
Negative Lookahead /x(?!y)/ matches a pattern (x) on condition that it is not followed by another pattern (y)
Positive Lookbehind /(?<=y)x/ matches a pattern (x) on condition that it is preceded by another pattern (y)
Negative Lookahead /(?<!y)x/ matches a pattern (x) on condition that it is not preceded by another pattern (y)
A key point in lookaround is the 2nd pattern (y), that is inside (?...), is never matched by the regex engine. In other words, lookarounds won't behave like captured groups with $1, $2, etc.
Enter input string
Rooms, bedsits, sharers £118.89 / $100.75, £84.79 / $75.90 Studios £187.50 / 150 USD, £123.80 / 100 USD 1 bedroom house/flat £265.12 / 250.35 USD, £160.36 / 140.50 USD 2 bedroom house/flat £375.70 £212.26 / 200 USD
Get all the numbers
Enter regex pattern
i - Ignore Case
g - Global Match
m - Multiline
s - Dot All
u - Unicode
y - Sticky
With lastIndex^?
/[0-9.]+/gCount:17
Rooms, bedsits, sharers £118.89 / $100.75, £84.79 / $75.90 Studios £187.50 / 150 USD, £123.80 / 100 USD 1 bedroom house/flat £265.12 / 250.35 USD, £160.36 / 140.50 USD 2 bedroom house/flat £375.70 £212.26 / 200 USD
match position
118.89 25
100.75 35
84.79 45
75.90 54
187.50 69
150 78
123.80 89
100 98
1 106
265.12 128
250.35 137
160.36 150
140.50 159
2 170
375.70 192
212.26 200
200 209
Get all the £ price values excluding the £ symbol.
Enter regex pattern
i - Ignore Case
g - Global Match
m - Multiline
s - Dot All
u - Unicode
y - Sticky
With lastIndex^?
/(?<=£)[0-9.]+/gCount:8
Rooms, bedsits, sharers £118.89 / $100.75, £84.79 / $75.90 Studios £187.50 / 150 USD, £123.80 / 100 USD 1 bedroom house/flat £265.12 / 250.35 USD, £160.36 / 140.50 USD 2 bedroom house/flat £375.70 £212.26 / 200 USD
match position
118.89 25
84.79 45
187.50 69
123.80 89
265.12 128
160.36 150
375.70 192
212.26 200
Get all the USD price values excluding the text USD.
Enter regex pattern
i - Ignore Case
g - Global Match
m - Multiline
s - Dot All
u - Unicode
y - Sticky
With lastIndex^?
/[0-9.]+(?= USD)/gCount:5
Rooms, bedsits, sharers £118.89 / $100.75, £84.79 / $75.90 Studios £187.50 / 150 USD, £123.80 / 100 USD 1 bedroom house/flat £265.12 / 250.35 USD, £160.36 / 140.50 USD 2 bedroom house/flat £375.70 £212.26 / 200 USD
match position
150 78
100 98
250.35 137
140.50 159
200 209
Get all the price values in USD and $ excluding the text USD and the symbol $.
Enter regex pattern
i - Ignore Case
g - Global Match
m - Multiline
s - Dot All
u - Unicode
y - Sticky
With lastIndex^?
/[0-9.]+(?=\sUSD)|(?<=\$)[0-9.]+/gCount:7
Rooms, bedsits, sharers £118.89 / $100.75, £84.79 / $75.90 Studios £187.50 / 150 USD, £123.80 / 100 USD 1 bedroom house/flat £265.12 / 250.35 USD, £160.36 / 140.50 USD 2 bedroom house/flat £375.70 £212.26 / 200 USD
match position
100.75 35
75.90 54
150 78
100 98
250.35 137
140.50 159
200 209
2. Nested and In-succession Lookarounds
Word Wrap
Lookarounds can be nested. This means you can place a lookaround inside another lookaround. In a lookaround pattern x(?=y), y can be another lookaround pattern like (?<=b)a. This can be written in a nested way as x(?=(?<=b)a).
Let's see this with an example. From the given list of positions, we are going to extract the male position titles only.
The pattern (?<=(?<!fe)male )\w+ gives the required output. In this pattern, the outer lookaround (?<=male )\w+ matches the titles that are followed by the word 'male'. To exclude female titles, we added an inner lookaround that matches the word 'male' that are not preceded by the string 'fe'.
Enter input string
List of Positions: Male Officer Female Doctor Female Officer Male Engineer Male Officer Female Engineer
Enter regex pattern
i - Ignore Case
g - Global Match
m - Multiline
s - Dot All
u - Unicode
y - Sticky
With lastIndex^?
/(?<=(?<!fe)male )\w+/giCount:3
List of Positions: Male Officer Female Doctor Female Officer Male Engineer Male Officer Female Engineer
match position
Officer 24
Engineer 66
Officer 80
The lookarounds can also be in succession to create a more complex pattern. Any two or more lookaround patterns such as (?=y) and (?<=b), can be written in succession as (?<=b)x(?=y) . We can get the same output for the above question by using two lookarounds in succession.
Enter regex pattern
i - Ignore Case
g - Global Match
m - Multiline
s - Dot All
u - Unicode
y - Sticky
With lastIndex^?
/(?<=Male )(?<!Female )\w+/gCount:3
List of Positions: Male Officer Female Doctor Female Officer Male Engineer Male Officer Female Engineer
match position
Officer 24
Engineer 66
Officer 80

match	position
118.89	25
100.75	35
84.79	45
75.90	54
187.50	69
150	78
123.80	89
100	98
1	106
265.12	128
250.35	137
160.36	150
140.50	159
2	170
375.70	192
212.26	200
200	209

match	position
Officer	24
Engineer	66
Officer	80

match	position
Officer	24
Engineer	66
Officer	80