Substitute a for B and B for a in a String

How to replace character 'a' with 'b' and 'b' with 'a' in a string?

You can do something like this:

var str = "blaah blaah blaah blaah";str = str.replace(/a|b/g, v => {       if(v=="a"){         return "b"       }else{         return "a"       }    });    console.log(str);

How to replace multiple substrings of a string?

Here is a short example that should do the trick with regular expressions:

import re

rep = {"condition1": "", "condition2": "text"} # define desired replacements here

# use these three lines to do the replacement
rep = dict((re.escape(k), v) for k, v in rep.iteritems()) 
#Python 3 renamed dict.iteritems to dict.items so use rep.items() for latest versions
pattern = re.compile("|".join(rep.keys()))
text = pattern.sub(lambda m: rep[re.escape(m.group(0))], text)

For example:

>>> pattern.sub(lambda m: rep[re.escape(m.group(0))], "(condition1) and --condition2--")
'() and --text--'

Javascript: Replace all occurences of ' a ' in a string with ' b ' with a regex

Use a lookahead after " a" to match overlapping substrings:

/ a(?= )/g

Or, to match any whitespace, replace spaces with \s.

The lookahead, being a zero-width assertion, does not consume the text, so the space after " a" will be available for the next match (the first space in the pattern is part of the consuming pattern).

See the regex demo

var regex = / a(?= )/g;var strs = ['x a y a x', 'x a a a x', 'x aa x'];for (var str of strs) { console.log(str,' => ', str.replace(regex, " b"));}

Substitute part of a word in a string by the number of characters

You may use gsubfn to be able to manipulate capture groups:

> library(gsubfn)
> gsubfn("\\y([a-z])([a-z]+)([a-z])", function(x,y,z) paste0(x,nchar(y),z),"this is a test string")
[1] "t2s is a t2t s4g"

Note that \y is a word boundary (equivalent of \b) in Tcl regex patterns (gsubfn uses Tcl regex library by default). You may also use perl=TRUE to use \b:

> gsubfn("\\b([a-z])([a-z]+)([a-z])", function(x,y,z) paste0(x,nchar(y),z),"this is a test string", perl=TRUE)
[1] "t2s is a t2t s4g"

Note that the capturing group values are passed to the anonynous replacement function via x (=\1), y (=\2) and z (=\3).

Best way to replace multiple characters in a string?

Replacing two characters

I timed all the methods in the current answers along with one extra.

With an input string of abc&def#ghi and replacing & -> \& and # -> \#, the fastest way was to chain together the replacements like this: text.replace('&', '\&').replace('#', '\#').

Timings for each function:

• a) 1000000 loops, best of 3: 1.47 μs per loop
• b) 1000000 loops, best of 3: 1.51 μs per loop
• c) 100000 loops, best of 3: 12.3 μs per loop
• d) 100000 loops, best of 3: 12 μs per loop
• e) 100000 loops, best of 3: 3.27 μs per loop
• f) 1000000 loops, best of 3: 0.817 μs per loop
• g) 100000 loops, best of 3: 3.64 μs per loop
• h) 1000000 loops, best of 3: 0.927 μs per loop
• i) 1000000 loops, best of 3: 0.814 μs per loop

Here are the functions:

def a(text):
    chars = "&#"
    for c in chars:
        text = text.replace(c, "\\" + c)

def b(text):
    for ch in ['&','#']:
        if ch in text:
            text = text.replace(ch,"\\"+ch)

import re
def c(text):
    rx = re.compile('([&#])')
    text = rx.sub(r'\\\1', text)

RX = re.compile('([&#])')
def d(text):
    text = RX.sub(r'\\\1', text)

def mk_esc(esc_chars):
    return lambda s: ''.join(['\\' + c if c in esc_chars else c for c in s])
esc = mk_esc('&#')
def e(text):
    esc(text)

def f(text):
    text = text.replace('&', '\&').replace('#', '\#')

def g(text):
    replacements = {"&": "\&", "#": "\#"}
    text = "".join([replacements.get(c, c) for c in text])

def h(text):
    text = text.replace('&', r'\&')
    text = text.replace('#', r'\#')

def i(text):
    text = text.replace('&', r'\&').replace('#', r'\#')

Timed like this:

python -mtimeit -s"import time_functions" "time_functions.a('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.b('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.c('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.d('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.e('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.f('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.g('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.h('abc&def#ghi')"
python -mtimeit -s"import time_functions" "time_functions.i('abc&def#ghi')"

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Replacing 17 characters

Here's similar code to do the same but with more characters to escape (\`*_{}>#+-.!$):

def a(text):
    chars = "\\`*_{}[]()>#+-.!$"
    for c in chars:
        text = text.replace(c, "\\" + c)

def b(text):
    for ch in ['\\','`','*','_','{','}','[',']','(',')','>','#','+','-','.','!','$','\'']:
        if ch in text:
            text = text.replace(ch,"\\"+ch)

import re
def c(text):
    rx = re.compile('([&#])')
    text = rx.sub(r'\\\1', text)

RX = re.compile('([\\`*_{}[]()>#+-.!$])')
def d(text):
    text = RX.sub(r'\\\1', text)

def mk_esc(esc_chars):
    return lambda s: ''.join(['\\' + c if c in esc_chars else c for c in s])
esc = mk_esc('\\`*_{}[]()>#+-.!$')
def e(text):
    esc(text)

def f(text):
    text = text.replace('\\', '\\\\').replace('`', '\`').replace('*', '\*').replace('_', '\_').replace('{', '\{').replace('}', '\}').replace('[', '\[').replace(']', '\]').replace('(', '\(').replace(')', '\)').replace('>', '\>').replace('#', '\#').replace('+', '\+').replace('-', '\-').replace('.', '\.').replace('!', '\!').replace('$', '\$')

def g(text):
    replacements = {
        "\\": "\\\\",
        "`": "\`",
        "*": "\*",
        "_": "\_",
        "{": "\{",
        "}": "\}",
        "[": "\[",
        "]": "\]",
        "(": "\(",
        ")": "\)",
        ">": "\>",
        "#": "\#",
        "+": "\+",
        "-": "\-",
        ".": "\.",
        "!": "\!",
        "$": "\$",
    }
    text = "".join([replacements.get(c, c) for c in text])

def h(text):
    text = text.replace('\\', r'\\')
    text = text.replace('`', r'\`')
    text = text.replace('*', r'\*')
    text = text.replace('_', r'\_')
    text = text.replace('{', r'\{')
    text = text.replace('}', r'\}')
    text = text.replace('[', r'\[')
    text = text.replace(']', r'\]')
    text = text.replace('(', r'\(')
    text = text.replace(')', r'\)')
    text = text.replace('>', r'\>')
    text = text.replace('#', r'\#')
    text = text.replace('+', r'\+')
    text = text.replace('-', r'\-')
    text = text.replace('.', r'\.')
    text = text.replace('!', r'\!')
    text = text.replace('$', r'\$')

def i(text):
    text = text.replace('\\', r'\\').replace('`', r'\`').replace('*', r'\*').replace('_', r'\_').replace('{', r'\{').replace('}', r'\}').replace('[', r'\[').replace(']', r'\]').replace('(', r'\(').replace(')', r'\)').replace('>', r'\>').replace('#', r'\#').replace('+', r'\+').replace('-', r'\-').replace('.', r'\.').replace('!', r'\!').replace('$', r'\$')

Here's the results for the same input string abc&def#ghi:

• a) 100000 loops, best of 3: 6.72 μs per loop
• b) 100000 loops, best of 3: 2.64 μs per loop
• c) 100000 loops, best of 3: 11.9 μs per loop
• d) 100000 loops, best of 3: 4.92 μs per loop
• e) 100000 loops, best of 3: 2.96 μs per loop
• f) 100000 loops, best of 3: 4.29 μs per loop
• g) 100000 loops, best of 3: 4.68 μs per loop
• h) 100000 loops, best of 3: 4.73 μs per loop
• i) 100000 loops, best of 3: 4.24 μs per loop

And with a longer input string (## *Something* and [another] thing in a longer sentence with {more} things to replace$):

• a) 100000 loops, best of 3: 7.59 μs per loop
• b) 100000 loops, best of 3: 6.54 μs per loop
• c) 100000 loops, best of 3: 16.9 μs per loop
• d) 100000 loops, best of 3: 7.29 μs per loop
• e) 100000 loops, best of 3: 12.2 μs per loop
• f) 100000 loops, best of 3: 5.38 μs per loop
• g) 10000 loops, best of 3: 21.7 μs per loop
• h) 100000 loops, best of 3: 5.7 μs per loop
• i) 100000 loops, best of 3: 5.13 μs per loop

Adding a couple of variants:

def ab(text):
    for ch in ['\\','`','*','_','{','}','[',']','(',')','>','#','+','-','.','!','$','\'']:
        text = text.replace(ch,"\\"+ch)

def ba(text):
    chars = "\\`*_{}[]()>#+-.!$"
    for c in chars:
        if c in text:
            text = text.replace(c, "\\" + c)

With the shorter input:

• ab) 100000 loops, best of 3: 7.05 μs per loop
• ba) 100000 loops, best of 3: 2.4 μs per loop

With the longer input:

• ab) 100000 loops, best of 3: 7.71 μs per loop
• ba) 100000 loops, best of 3: 6.08 μs per loop

So I'm going to use ba for readability and speed.

Addendum

Prompted by haccks in the comments, one difference between ab and ba is the if c in text: check. Let's test them against two more variants:

def ab_with_check(text):
    for ch in ['\\','`','*','_','{','}','[',']','(',')','>','#','+','-','.','!','$','\'']:
        if ch in text:
            text = text.replace(ch,"\\"+ch)

def ba_without_check(text):
    chars = "\\`*_{}[]()>#+-.!$"
    for c in chars:
        text = text.replace(c, "\\" + c)

Times in μs per loop on Python 2.7.14 and 3.6.3, and on a different machine from the earlier set, so cannot be compared directly.

╭────────────╥──────┬───────────────┬──────┬──────────────────╮
│ Py, input  ║  ab  │ ab_with_check │  ba  │ ba_without_check │
╞════════════╬══════╪═══════════════╪══════╪══════════════════╡
│ Py2, short ║ 8.81 │    4.22       │ 3.45 │    8.01          │
│ Py3, short ║ 5.54 │    1.34       │ 1.46 │    5.34          │
├────────────╫──────┼───────────────┼──────┼──────────────────┤
│ Py2, long  ║ 9.3  │    7.15       │ 6.85 │    8.55          │
│ Py3, long  ║ 7.43 │    4.38       │ 4.41 │    7.02          │
└────────────╨──────┴───────────────┴──────┴──────────────────┘

We can conclude that:

• Those with the check are up to 4x faster than those without the check

• ab_with_check is slightly in the lead on Python 3, but ba (with check) has a greater lead on Python 2

• However, the biggest lesson here is Python 3 is up to 3x faster than Python 2! There's not a huge difference between the slowest on Python 3 and fastest on Python 2!

Java: replacement in a string according to multiple rules

Here is an algorithm you can use:

Assumption: String consists of only (A,B,C)

If string is composed of B's only (no A/C), output = input string.

Otherwise:

Divide the string in the following way. If a substring consists of (B,A) or (B,C), divide them. Replace with A and C respectively. That will be the answer.

eg. Let the string be: "BBBAABABBBCCBCBBCACB". This will be divided as:

"BBBAABABBB" "CCBCBBC" "A" "CB"

And this will result in output string as: ACAC

Basically, just ignore all B's and replace clusters of A's with A and clusters of C's with C.

How to replace two things at once in a string?

When you need to swap variables, say x and y, a common pattern is to introduce a temporary variable t to help with the swap: t = x; x = y; y = t.

The same pattern can also be used with strings:

>>> # swap a with b
>>> 'obama'.replace('a', '%temp%').replace('b', 'a').replace('%temp%', 'b')
'oabmb'

This technique isn't new. It is described in PEP 378 as a way to convert between American and European style decimal separators and thousands separators (for example from 1,234,567.89 to 1.234.567,89. Guido has endorsed this as a reasonable technique.

How to use replace(char, char) to replace all instances of character b with nothing

There's also a replaceAll function that uses strings, note however that it evals them as regexes, but for replacing a single char will do just fine.

Here's an example:

String meal = "Hambbburger";

String replaced = meal.replaceAll("b","");

Note that the replaced variable is necessary since replaceAll doesn't change the string in place but creates a new one with the replacement (String is immutable in java).

If the character you want to replace has a different meaning in a regex (e.g. the . char will match any char, not a dot) you'll need to quote the first parameter like this:

String meal = "Ham.bur.ger";

String replaced = meal.replaceAll(Pattern.quote("."),"");

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